What is a difference between not focus and blunder?

Depends on who you ask to. Not focused to me is when you hang pieces because you are tired, have been playing blitz all day etc... Blundering is when you hang stuff when you are clearly not tired, focused and have a clear mind.

A "blunder" is a characteristic of a move, not of the circumstances the move was made in. For example, in the line 1. e4 e5 2. Bc4 Bc5 3. Qh5 Nf6?? the last move is a blunder, as it simply misses mate in 1. It doesn't matter if you made this move because you were not focused, or because you were premoving it expecting a conventional play from white, or because you are a complete newcomer and miss such obvious threats still - a blunder is a blunder.