Mach is unit of measure used to describe the high speeds used during sub-orbital flight, and is equal to the speed of the object divided by the speed of sound.
When the mining shuttle Nenebek passed through the atmosphere of Lambda Paz in preparation for a crash-landing in 2367, the crew had to manually trip the bypass from the main engine to the thrusters to maintain a controlled entry, which was performed when the shuttle had slowed to below Mach 1. (TNG: "Final Mission")
When the orbital tether used to escape a Nezu colony in 2373 had reached the upper troposphere it was traveling at a velocity of Mach 2.1. (VOY: "Rise")