Latency
The delay between sending data and it arriving, usually measured as a round trip in milliseconds.
Bandwidth determines how much can move; latency determines how quickly a conversation happens. Applications that make many small requests, such as a database over a VPN, feel slow on a high-latency link regardless of available bandwidth.
It is why adding bandwidth often fails to fix a slow application, and why moving a server further from its users can hurt more than expected.
Where this comes up
Latency sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.