are measured in millions, so 25,083,000 people heard it yesterday on the radio.
On the Hot 100 1,000,000 Audience Impressions are equivalent to 1,000 paid downloads/sales, and the tracking weeks for these are as follows:
Monday, January 1 – sales tracking-week begins
Wednesday, January 3 – airplay tracking-week begins
Sunday, January 7 – sales tracking-week ends
Tuesday, January 9 – airplay tracking-week ends
Thursday, January 11 – new chart released, with issue date of Saturday, January 20.
The dates are just there as an example, but don't forget that the airplay updates we get are always from the previous day so the last update counting towards the Hot 100 this week will be Wednesdays.