tempo primo
Return to the very first tempo of the piece.
In Depth
Tempo primo means first tempo — an instruction to return to the very first tempo marking of the piece, regardless of any changes that have occurred since. It is more specific than a tempo, which returns to the most recent tempo rather than necessarily the opening one. The marking is particularly useful in pieces with multiple tempo changes. After a slow middle section in a fast piece, tempo primo explicitly tells the performer to go back to the original speed of the opening. It ensures clarity when a simple a tempo might be ambiguous about which previous tempo to restore.
In Mozart's time, tempo primo was so important that failing to return to the original speed was considered a sign of an amateur performer.