From an accounting manual for a university - with many definitions for various payment terms I found this:
2% 10 and 25th =
2% discount allowed on payments made on or before the 25th of the month for invoices issued the first half of the month; 2% discount allowed for payments made on or before the 10th of the succeeding month for invoices issued the last half of the month.
(an accounting manual, the Disbursements: Discount Terminology section)
http://www.ucop.edu/ucophome/policies/acctman/d-371-23.pdf.