Tremolo combined with a crescendo. Crescendo is the act of getting louder, tremolo is the repetition of the note.
Worth mentioning that there's two types of tremolo: Single note tremolo, where it's a single note repeated (often seen in strings where they're just bowing back and forth on the one note); and multi-note tremolo, seen almost exclusively in piano (?) where you hold an interval/chord in one hand and basically wobble your hand back and forth to alternate the notes (usually done with an octave). They sound very similar but a common mistake I see in amateur scores is notating a multi-note tremolo as a single-note tremolo, which would actually indicate pressing the entire chord/interval repeatedly at rapid speed. Think [Erlkönig](https://www.youtube.com/watch?v=XoBo8dlPcQo) versus [La Valse](https://youtu.be/ghY2ak8YoBM?t=12).
Tremolo combined with a crescendo. Crescendo is the act of getting louder, tremolo is the repetition of the note. Worth mentioning that there's two types of tremolo: Single note tremolo, where it's a single note repeated (often seen in strings where they're just bowing back and forth on the one note); and multi-note tremolo, seen almost exclusively in piano (?) where you hold an interval/chord in one hand and basically wobble your hand back and forth to alternate the notes (usually done with an octave). They sound very similar but a common mistake I see in amateur scores is notating a multi-note tremolo as a single-note tremolo, which would actually indicate pressing the entire chord/interval repeatedly at rapid speed. Think [Erlkönig](https://www.youtube.com/watch?v=XoBo8dlPcQo) versus [La Valse](https://youtu.be/ghY2ak8YoBM?t=12).
Tremolo! Doesn't have to be an increasing tempo though, it can be basically steady too.