The rate of data transfer. It may refer to both the rate what tor requires to function, and also the data rate of the embedded traffic.
It is a measure of how much data can be transferred over a medium in a given amount of time, mostly shown as kBit/s or MBit/s. It is the primary way of classifying the speed of data exchange.
They incoming bandwidth ("download speed") and the outgoing ("upload speed") typically differ.