"Atheist" and "theist" are terms somewhat similar to "homosexual." People who are atheists or theists or homosexuals (or white or any other label people give themselves) use it to mean a specific thing but the next person using that label may not agree. Their social definitions are more community based than belief based. "Homosexual" really means "I associate/identify with other people labeled homosexual." Nothing gets everyone with a particular label riled up faster than asking them to define who is or is not part of the club.

Someone somewhere came up with a term that seemed fairly obvious but once it turned into a label any accuracy went out the window.

More specifics with the examples I used above:

• Atheist: Not believing that God exists or believing that God does not exist? Or just not knowing?
• Theist: Believing God exists; believing a god exists; this is less tricky but it begins to get weird when including things like Maoism or panantheism
• Homosexual: Someone who has sex with the same sex; someone who has sex with the same gender; someone attracted to the same sex/gender; someone who is only attracted to the same sex/gender; other variations
• White: Light-skinned; light-skinned but not hispanic; anyone who has white "blood"; not-Black; Europeans and their descendants

(Note) This is off the cuff. The specifics are just there to illustrate the point. If I need to change them let me know.