chess terms

Blunder is a term for a bad mistake - usually one that is game changing - losing your Queen or a rook for nothing in return or allowing a checkmate in 1, that sort of thing

'a move which changes a winning game into a losing one deserves two question marks' - Hübner

A "swindle" is when you turn an expected loss into a win or a draw.

A blunder is a specific type of mistake which typically risks the IMMEDIATE net loss of material or worse - the threat of imminent checkmate of the blunderer.  Whereas an inaccuracy, while it technically could be considered a mistake, is not immediately harmful and typically does not risk the loss of material or checkmate, at least not in the short term.  So it's blunders that you must absolutely try to avoid.  The best way to train yourself to avoid making blunders is to do a blunder check of your completed games.  The chess.com post-game analysis function will identify and indicate the blunders, inaccuracies and any other mistakes in your completed games.

Improving Your Chess - Resources for Beginners and Beyond.....
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell

As far as the engine is concerned, a blunder is a move that decreases the position by a substantial number of points in the opponent's favor. If your opponent blunders, there is an opportunity to win a piece, and the position jumps in your favor. If you miss the blunder, then you have also committed a blunder, as theoretically, your opponent could take back the piece he blundered, thus leading to you losing points in the position. Ninety nine times out of a hundred, blunders are of a tactical basis. 

A mistake is a move that decreases the position by a more moderate number of points in your opponents favor. This can still be a tactical issue (hanging pawns, losing the exchange), but there is a great deal more positional considerations that lead to a mistake (doubled pawns, seizing of weak squares, establishing outposts, etc.)