I have some sheet music for vocal + piano accompaniment where I want to notate that the vocalist should feel out the rhythms and phrases and sing expressively while the pianist maintains a steady tempo. I'm concerned marking it simply "rubato" might be misleading and the pianist may think they're supposed to follow the vocalist. Is there a better way to notate that or is it enough to notate the tempo as "Expressively" or "Rubato" (plus a bpm) and assume that, if the piano part is rhythmic (mostly 8th notes in 4/4), the pianist will understand they should stick to the beat?
I posted this as a comment but @Bob_Broadley suggested I post it as an answer so here goes:
This is pretty wordy but sometimes using a verbal description is the best way to go. There is no rule that says musical instructions must be only one or two words long. How about: “vocalist ad-lib rhythm over steady piano accompaniment” ?
This happens all the time in popular music. Streisand rarely ever DOESN'T do it!
If it's not obvious from the style, give plain text instructions. 'Maintain rhythm, vocalist may float'.
Best not to include the literal melody in the accompaniment if this is going to happen!
This can be a bugbear when accompanying at sight. It isn't always clear whether the singer wants to play around rhythmically over a steady beat, or wants 'colla voce'. I've had sticky moments over this as a club/audition pianist!