Ronnie's reply seems to be the closest to what I think a strong atheist is. George Smith in his book, Atheism: The Case Against God presents a breakdown between weak and strong atheism. Essentially, I think strong atheism is a positive belief that a god or maximally great being does not exist. With that positive belief, comes a burden of proof for the atheist. The atheist can't rely on cute anecdotes to make his/her case...."You tell me why you don't believe in Thor and then I'll tell you why I don't believe in your god...". That is not a strong atheist answer.
Also, the strong atheist can't really make a case with lame premises like "I'm an atheist because religion just doesn't make sense or is false". Being negative about theism, does not rationally prove anything about atheism. All you've done is critiqued theism. A strong atheist needs to provide positive arguments for why s/he believes that atheism is correct....warranted, justified.
And the silly idea that atheism is a lack of belief is not representative of strong atheism. The trouble with the lack of belief argument is that it lacks definition every time and is not specific enough to make rational sense. How can one have a lack of belief in an idea that s/he is promoting? The atheist believes that a god of MGB does not exist. How can a so-called rational person claim a belief that god does not exist and then claim a lack of belief in a god? I think I understand what the atheist is trying to do in these cases, but it sounds very illogical and poorly defined. Theory of knowledge 101. Knowledge is defined as justified, true belief.
I enjoyed his book and relate to a lot of his ideas. I dislike the term of an atheist as having a "lack of belief". The positive belief that any god does not exist is where I fit as a strong (gnostic) atheist.
I tend to define myself as "Anti-theist". I guess that the simplest explanation of what I mean is that I don't care I a God/Gods/Goddess, or what ever reviled themselves to have created the universe, and all in it, I would STILL not submit to worshipping him/her/them or it. As the probability of any form of higher being, and the underlaying question of their evolution/creation, is so extremely improbable (and down right bland), being against such a divisive and damaging meme as theism is the intelligent way to go.
Personally, I describe my 'strong atheism' by how I do not entertain any unprovable supernatural claims, how I am open and vocal about my lack of belief in a god or deity and I also like to list my antitheism as a part o my being a strong atheist.
Of course, I don't think these are the standards everyone should follow, should there be any standards for defining what a 'strong' atheist is at all.
Expounding on my first few statements, when I say that I do not entertain and unprovable claims, I mean that 'supernatural' things must first have some evidence to support it really is 'supernatural' and I will not believe it until it does. I do not give ghosts or magic or miracles the 'benefit of the doubt' (if that is the term) and entertain the possibility that they may be unearthly or unexplainable by science. To me, these things must have an explanation and even without an scientific explanation, I would still need proof to believe in it's claims.
When specifically speaking of belief in a god, I think that there is not only absolutely no proof to support the existence of a god or deity, but I also think that there is a lot of proof AGAINST the various concepts of gods from religion to religion.
I think my second statement doesn't need much explaining, but on my position of antitheism, I believe that there is nothing positive that you can attribute to religion that you cannot also attribute to secularism. I'll even go a step further as to say that organized religion is harmful and vile and might as well be the root of most problems in the world.
I know it can be said that there have been good deeds done in the name of religion, but again, seeing as I don't see them as deeds exclusively religious, I don't see their good deeds as even coming close to equaling the horrors and atrocity of religion.
I think a strong atheist is someone who firstly is quite willing to declare themselves as an atheist. Is someone who doesn't accept that we should automatically respect a person's religion or accept that religious people should have special privileges. Is someone who knows the arguments against theistic beliefs and will argue those points. And also I think a strong atheist is someone who gets dismayed and often angry when they see the dreadful things that are done in the name of religion.
The whole "atheist" term is often debated. Sam Harris doesn't like it. Richard Dawkins has argued against it. Personally I think it is a term that has come to mean something more than just its literal linguistic form. I think it it is a term that has been reclaimed by free-thinkers. Similar in the way the word "gay" is used. I think Wittgenstein could probably explain it better than I can.
Some say it is a negative position but Dan Barker says that as religion is negative to begin with, and that he considers a-theism to be a double negative: which gives a positive. Nice one Dan.
It's possible that the word "strong" might need some definition. I hope this doesn't harken back to Bill Clinton stating "It depends on what "is" is".
Taking a philosophical point of view, "strong" atheism could be "anti-theistic", considering religions to be bad, not just incorrect. ie, "religions have harmful effects and I am against anything that promotes religion".
Or it could mean having a "strong degree of certainty" that life originates from chemical and biological processes as a result of physical laws, and that history occurs as the unfolding of human interactions with one another and their circumstances (ie, "Based on scientific studies, biochemical research, paleontology, archeology, and history, which provide non-metaphysical explanations for why we are where/who we are today, I'm CERTAIN there are no gods as we have defined them"). I try to phrase in the positive since you can't really prove a negative"
Or it could mean "strong proponent" of atheism, as in "not only am I atheist, but "you" should be too".
I view myself as "strong" on the first 2 senses, and I feel a sense of general responsibility in the 3rd. However, in my workplace and professional life, I have to tolerate the religious practices and beleifs of others due to "fair workplace" regulations. Plus in the interest of working with people are the best at what they do, personal beleif is not relevant (nor is race, gender, age, national origin, sexual orientation, marriage status...). In addition, though, I don't hide my "pro-science" candle under a bushel, either.
I think a good definition would be a combination of all three, with individuals having a stronger emphasis of certain points over the other, but with the agreement that all three are important. Personally, I emphasis the first two, with the hope that education and evidence will force the third.
Strong Atheist - a person who is unwavering in their understanding that gods do not exist.
I am being specific with my definition in that I do not dispute the rather wishy-washy stuff some people use in reply, "but there are some things that man doesn't fully understand yet." Sure, we are working on some details. However, 'there are no Gods' is a clear statement of fact. To say that there might be a God or Gods so we shouldn't be so sure of our position, is rubbish. There is no evidence at all in favor of an invisible being who concocted everything from quarks to galaxies and set it all in a prearranged motion. There are no muscular ghosts inside our planet that push it about its axis but by the theist argument, you cannot deny their existence because they might exist.
I borrowed the muscular ghosts concept from an excellent book, The Atheist Debater's Handbook by B.C. Johnson, published by Prometheus Books. Another interesting book is entitled Santa Lives! by Ellis Weiner published by Riverhead Books. This slim volume recreates five common arguments for the existence of a God or Gods but with Santa substituted for God. Guess what? The arguments are just as persuasive and just as flawed. If someone argues for God, they must equally accept Santa Claus.
There is also a hint of in-your-face atheism inherent in the word "strong." I do class myself as a strong atheist but not an aggressive Atheist. An Evangelical atheist? An interesting oxymoron. As I get older, I have less tolerance for the great deluded masses and have a growing tendency to tell them what a load of crap their silly beliefs are, especially as it becomes apparent that the fate of the planet may hinge on their silliness versus the rational approach of science.
I define myself as a strong atheist because first I don't want to have anything in common with the most powerful mean of propaganda that religion is. I hate propaganda. Also, I am very much attached at the idea of technical and social progress and religion holds us back to Middle-Age. I also want to think by myself and I have an independant mind. To give you an example, even the politicians I consider myself the closest of can be criticized by me and I want by all means to be free to think the opposite of what they say if I want to. It is absolutely not compatible with the dogma of any church that I would have to follow unconditionally.