terminology - What do you call a phrase where one part is fixed and another part is free?

What do you call a phrase the where overall phrase structure (i.e. grammar) plus the use of a few fixed words makes it recognisable?

E.g. "raining cats and dogs" could also be "pouring cats and dogs" (although this is a very poor example).

The structure in this example is "[verb]-ing cats and dogs", where "[verb]" can be any verb that is a synonym of "to rain", meaning that the phrase is not totally fixed but does allow some small variation.