A musical direction to return to the originally stated tempo.
In this example, the music is stated to be at a tempo of Allegro. Within the music, a Rallentando begins within the third bar. Without the A Tempo marking the tempo reduction would have continued into the next line (5th Bar). However, the A Tempo marking tells us to go back to the Allegro tempo in the next line (5th Bar) until the end of the piece.
"A Tempo" Example with Metronome:
"A Tempo" Example without Metronome: