There are several concepts in mathematics for which informal definitions are a dime a dozen but formal definitions seem to elude normal search methods. This website exists as a sort of dictionary for those who wish to find a clear-cut, mathematical definition of a term which seems wishy-washy at best. Each definition contains citations for further research.
If you think I should add a specific word to the dictionary, contact me at isissquillante [at] gmail.com and I will try to add an entry. Please try to find the formal definition of a term on your own before coming to me; if it is easily findable without much searching, I will be reluctant to add it to the database.