For our Racecasts, we give a probability of precipitation (PoP) that covers a 12 hour period (from 7 am to 7 pm) at a 5 mile radius around the race track. Essentially, it is a way for us to express how confident we are that rain will occur in that circle.
We also use different wording in our forecast to express the chance of rain. Here are the phrases that we use that correspond to different ranges of PoP:
15-24% = slight chance of rain
25-54% = chance of rain
55-74% = rain likely
75-100% = definite rain