People have been trying to redefine the word "atheist" to suit their own agendas. Some christian groups are insisting that an atheist would have to be someone with absolute knowledge to know there is no god. Some atheist groups are insisting that it means anyone who would answer "anything other than yes" when asked if there was a god. Both sets of groups may have convinced themselves internally. In general usage, however, it means someone who believes there is no god. The word does not require absolute knowledge, nor does it include anyone who hasn't come to a (tentative, at the very least) decision.
The claims of the christian groups on the matter are so ridiculous that I cannot imagine anyone who actually thinks about them to accept them. I shall, therefore, not deal with them here.
The claim of the atheists groups sound reasonable until you look more closely. So, I would like to address them. They primarily come down to a claim that an atheist is someone who "doesn't believe in a god," so must include the undecideds. Well, an atheist someone who doesn't believe -- in the sense of someone coming back with a report on the results of an announcement "they don't believe us." No one, on hearing such a report, would decide that the people hadn't decided whether the announcement was true. That report conveys the sense that people believe the announcement false. An atheist is someone who doesn't believe there is a god like I don't believe there is a Santa Claus. I believe there is no Santa Clause; and an atheist believes there is no god.