It's just an asymptotic upper bound (though often used to imply tight bounds). It's most commonly applied to worst case running time, but is often applied to e

It's just an asymptotic upper bound (though often used to imply tight bounds). It's most commonly applied to worst case running time, but is often applied to expected running time ("hash table insertions run in O(1)"), space complexity, communication overhead, numerical accuracy and any number of other metrics.