In the 2019 laws, a mark is referred to in Law 17:
To claim a mark, a player must:
Have at least one foot on or behind their own 22-metre line when catching the ball or when landing having caught it in the air; and
Catch a ball that has reached the plane of the 22-metre line directly from an opponent’s kick before it touches the ground or another player; and
Simultaneously call “mark”.
In the 2017 laws, the wording included a reference to a "clean catch"
To make a mark, a player must be on or behind that player’s 22-metre line. … The player must make a clean catch direct from an opponent’s kick and at the same time shout “Mark”.
The laws don't appear to include a definition of mark, only some necessary conditions. (For example it doesn't say that a mark must be taken in the field of play.) But the omission of 'clean' seems to be strange. Is it an intentional omission?