Megabits per second

A measure of data transmission rate (speed) equal to approximately 1 million bits per second.

Data rate is also often measured in Kbps or Gbps:

1 Mbps = 1024 Kbps
1 Gbps = 1024 Mbps

The "Mb" in Mbps should not be confused with MB, which stands for megabytes, a very different kind  measurement.