[astine]

The term for this is a "stipulative definition." [1] It's only a fallacy if one does not clarify that one is using a stipulative definition instead of a more common one.

1. https://www.thoughtco.com/stipulative-definition-1692143

[dragonwriter]

Even if one is clear about the definition in use, it's the fallacy of equivocation if it is used to counter an argument using a different definition.