What is the official name for melodic phrases that only come in a song every 4, 8 or 16 bars? You usually hear them transitioning into another bar....

 

Motif? Theme? Ostinato? Subject? Riff? Hook?... Not sure if you're setting us up for a punch line or an honest question...

 

Fill or fill in ?

https://en.wikipedia.org/wiki/Fill_(music)

 

interesting. i don't think there is any 'official' name. other than the slang terms like 'sting', 'hook', "the bit before the other bit", etc. is there anything official in music?

from both a constructive and analytic perspective in relation to expanded traditional rondo forms. i would say they fulfill the cadential function of the consequent phrase to tie larger thematic movements together, and provide forward momentum while maintaining cohesion.

so in a one pseudo-official word, 'consequent' phrase -- maybe. i don't know.

 

It is called a "melodic fill".