Expectation
Also: expected value, mean
The long-run average of a random quantity, each outcome weighted by its probability.
What people get wrong
It need not be a value that can actually occur, and on skewed data it is a poor summary — which is why latency is reported in percentiles rather than as an average.
A definition is not an understanding. This is taught properly in The distributions you will actually meet, part of Probability & statistics.
Learn this properly