Positive EV
A bet is positive EV when the price on offer pays more than the true chance of the outcome warrants.
In plain English
The phrase describes the price relative to reality, not confidence in the result. A positive-EV bet still loses often, and a string of losses does not by itself show the assessment was wrong.
Example
If a fair price would be +250 and a book offers +300, backing it is positive EV even though it will lose most of the time.
Why it matters
Chasing likely winners and chasing good prices are different strategies, and only one of them survives the vig.