The Definitions and
A simple popular definition of a Mechanical Puzzle may be: -
Any object that you can give to someone with a question starting "Can You ... ?".
Thus:- Can you put it together? Can you take it apart? Can you Drink out of it? Etcetera.
To solve one of these puzzles rarely requires any special knowledge, language or education. Everyone starts at an equal level as all the information required in the solution is available in the puzzle itself.