Ostinato

The key feature is persistent recurrence. A bass playing C-G-A-G every bar is an ostinato even if the melody above changes. It need not be identical down to every accent, but the listener should recognize the cycle.

A stable pattern gives the ear a reference. When a new vocal line, chord or drum fill enters, the repeating part makes that change easier to notice. It can also create momentum without requiring a new chord on every beat.

No. A two-note guitar figure, an arpeggio on a synthesizer or a repeating hand-clap rhythm can serve the same role. 'Ostinato' describes the repetition, not the instrument or register.

A leitmotif is tied to a character or dramatic idea and may return only at selected moments. An ostinato is identified by ongoing repetition. A motif could be both if it repeats insistently and carries a story association.

Try removing the pattern just before a chorus or final line. The silence can make the next entrance stronger. If the ostinato's contour competes with a singer's phrase, simplify one or move it to another register so both lines remain clear.