There will often be words that academics use differently from laymen. For example, a lot of social scientists have a take racism to mean that, by definition, says an underprivileged class cannot be racist. The layman definition is simpler but varies between people. But universally, to any layman, racism carries a strong, personal, and negative stigma that is different from "prejudice". Being called "racist" is very much an insult. Disallowing blacks to be racist, then, provides a kind of immunity for black people, as being "prejudiced" doesn't leave the same bad taste in your mouth. So it is not entirely productive to insist on a definition that isn't widely accepted during a conversation, especially when your definition does not preserve fairness in discussion.
The situation is similar in the religion debate. The atheist community and philosophical circles along with laymen disagree on the terms "atheist" and "agnostic". To the atheists, the word "atheist" simply means those who do not believe in God. However for a long time the word "atheist" has meant, to both philosophers and laymen, strictly a person who disbelieves in God, with the word "agnostic" being reserved for those who hold no belief either way. It was in 1972 that philosopher Antony Flew attempted to change the definition of atheism to the broader sense. On this, Uri Nodelman, editor of the Stanford Encyclopedia of Philosophy, writes:
Not everyone has been convinced to use the term in Flew's way simply on the force of his argument. For some, who consider themselves atheists in the traditional sense, Flew's efforts seemed to be an
attempt to water down a perfectly good concept. For others, who consider themselves agnostics in the traditional sense, Flew's efforts seemed to be an attempt to re-label them "atheists" -- a term they
rejected.
Basically, Flew's definition is favorable to the atheist community, as it labels more people "atheists". But in the view of those who claim themselves to be agnostic in the layman's way, this definition takes away the ability to distinguish themselves from their atheists -- i.e. they lose expressibility. As such, the atheist community's definition of atheism is for the most part unused.
We see another example in the feminist movement. According to feminists, a feminist is simply one who believes in equal rights for men and women. But plenty of people hesitate to call themselves feminists -- and if such a person happens to be a celebrity, or even a female celebrity, they are scorned. It is no wonder, then, that the public's definition of feminism differs from their own. To the public, feminism means actual advocacy. The word refers to a movement, not an idea. However, it is advantageous for the movement itself that the word associates to the idea, since their movement would receive a free upgrade from a controversy-stirring movement to an idea impervious to assault.
The problem is that definitions aren't constructed by textbooks or academics. Formal definitions are post hoc. That is, definitions are written down and systematized only after a large part of society actually starts using the word in some regular fashion. That definitions are innately human, and that definitions are determined by layman consensus is essentially a tautology -- it's simply how words are used. It's not particularly right or wrong, it's just that to attempt to write down your own definition to suit your needs and trying to get everyone else to conform to it by saying "because it's right" is missing the forest for the trees.
We have an analogy in music. Music was written, and existed, long before music theory. Much like dictionary definitions, music theory is post hoc, a fact that is emphasized in music theory courses. Yes, theory provides a nice standard, but it doesn't make sense to preach theory to the thousands of brilliant musicians who don't know how to read music. Additionally, it would make much less sense to construct a music theory that disagrees with how the majority of musicians see music. Science seeks to explain the world via theory. Certain parts of social science do the opposite -- they attempt to conform the world to their theory. Perhaps this is why many do not regard social science as a true science?
In the case of atheists and feminists, it can be argued that their change-of-definitions serves mainly to facilitate communication amongst themselves and further their own groups' cause. From their perspective, this is inherently good and useful. However this actually changes the dynamic of the discussion, by disenfranchising those who wish to distinguish themselves from the group despite not opposing them completely.
In short, adopting a definition does not preserve the status quo. Change-of-basis can add to or detract from expressibility or nuance, and so essentially change the power dynamic in social dialogue.
