Tempo is at the pleasure or discretion of the performer. The same as ad libitum. Similar to rubato except rubato is the discretion of the conductor.

Accelerando (accel.)

Quickening; a gradual speeding up of the tempo

Alla Breve

Duple time with a half note as the unit; same as Cut time

Allargando (allarg.)

Broadening; sometimes interpreted as a combination of ritardando and crescendo

ancora

Still more; used generally with tempo indications, as ancora meno mosso (still more slowly)

A tempo

In tempo; reestablishes original speed after accelerando or ritardando

cédez

French for poco ritardando; a slight holding back

Come prima

As at first

Come sopra

As above; much the same as come prima

Doppio movimento

Twice as fast; usually used in conjunction with Common time changing to Cut time

L'istesso tempo
lo stesso tempo

The same tempo, indicating that the beat remains constant when the meter changes as 2/4 to 6/8

Meno mosso
Più lento

Less motion; slower. May be qualified by poco (a little less)

Più allegro
Più animato
Più mosso

faster

pressez

French term for "quicken"; same meaning as accelerando

Rallentando (rall.)

Same as ritardando; a gradual slowing-down of the tempo

Ritardando (rit.)

Slowing down; a gradual holding-back of the tempo

Ritenuto (riten.)

Frequently confused with ritardando; means a sudden slowing-down, in contrast to the gradual holding-back of rallentando

rubato

Litterally, to "rob" or steal the time values by holding-back or speeding up at will to color a phrase

Stringendo (string.)

Pressing forward; sometimes erroneously interpreted as a combination of accelerando and crescendo

Tempo giusto

In strict tempo (non-rubato)

Tempo primo (Tempo I° )

Original tempo

Trattenuto (tratt.)

Holding back (same as rallentando) with the addition of a sustained quality (sostenuto)