What does 15ma mean in Piano Theory?

Answer 1

[It., quindicesima]

Abbreviation for quindicesima or "at the fifteenth." This indication is found above specific notes on a staff and indicates that those notes should be performed two octave higher than written. This indication can be used with a dotted line that covers a series of notes to be performed at the fifteenth. The dotted line should end with a downstroke to indicate the end of the passage to be altered. The end of the passage can also be indicated by the term loco ("at place") or perform at the written pitch.

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http://www.music.vt.edu/musicdictionary/text1-9/15ma.html

Answer 2

This indication is found above specific notes on a staff and indicates that those notes should be performed two octave higher than written.