Let me give specific examples of what I consider flaky behavior:
Me: So, when are you going to cook me your famous lasagne?
Girl: I off this Sunday maybe we can meet up at Purdy or Standard.
Me: No, maybe on my part, lets meet at Standard.
Girl: Ok, Deal!
Sunday early in evening I text her about what time she wants to meet up - no response. I write her off. 11:00pm I get a text from her: I'm meeting up with my crew. I think we are going to hit Jazzid, not sure.
Notice she never apologized or even mentioned that she had agreed to meet at Standard and this is not a girl I just met and got her phone number. I took her home and the only reason I didn't bang her was because she had a girlfriend staying with her in her apartment which I didn't know before hand. So this is a girl that I had already spent quite a bit of time with.
A flake is when a girl agrees to meet you and then changes her mind for whatever reason.
Another example:
Girl: let's meet after work on Thursday or Friday.
Me: Friday works better for me. I will text you where on Friday afternoon.
Girl: Ok
Friday afternoon I text her the place. She replies: I am really tired from work not going to go out tonight. Let's meet another day.