Bandwidth is the measure of the bit rate (bits/seconds) that a specific link device (e.g., a router) can send or receive over a network.
It is worth noting that bandwidth is not a measure of speed, but capacity (theoretical maximum bit rate of the network). Therefore on a typical shared bandwidth network, the network speed is equal to the bandwidth only if there is only one active user transmitting data over the network. If there are multiple active users, then the network speed is the bandwidth divided by how many active users there are. The bandwidth is the pipe and speed is the stream. So technically the bandwidth of a 300 Mbps home internet connection is not 300 Mbps since many households nearby are sharing the same connection with a much larger bandwidth to accommodate the collective internet usage; the ISP artificially limit the speed of home internet connections to sell different tiers of speed. The shared network bandwidth may not support all active users using their full allowed speed simultaneous, which means throttling is required at times.
Bandwidth is originally a DSP term that refers to the difference between the minimum and maximum frequencies used by the carrier to transfer data, from which the maximum data rate can be inferred. The term data rate / transmission rate has become synonymous with bandwidth lately since the two are very closely related.