How do you define a trilogy?

Or what is the accepted definition of such?

Is it strict like Back to the Future? 3 movies released in succession with the same characters/plot?

Or does any 3 movies like The Godfather also count regardless when released? Even though no one counts III.

Or do you count the first 3 realeased Star Wars? And consider the rest separately?

What about Indiana Jones/Mad Max? 3 great films, then a 4th that was sad/fun.

Man with No Name Trilogy? Same director, actor, theme, etc. But not really linked together.

Does Dark Knight count? Multiple Batman movies, but the DK 3 were their own thing?

Thoughts?