IPA: /ˈrɪðmɪst/
KK: /ˈrɪðmɪst/
A person who is skilled in understanding or creating rhythm, especially in music or dance.
The rhythmist led the group in perfect time during the performance.
Rhythmist is formed from "rhythm" (from Greek "rhythmos", meaning measured flow or movement) and "-ist" (meaning one who practices or is concerned with). The word refers to a person who is skilled in or practices rhythm.
Think of someone who is skilled in 'measured flow' ('rhythm') — that's what a 'rhythmist' is.