The term 'BILL ABBR.' is an abbreviation for three different terms related to billing. The first is AMT, which stands for amount. This is the total amount of money that is due for a particular bill. The second is INV, which stands for invoice. This is a document that outlines the details of a transaction, including the amount due and the date it is due. The third is ACCT, which stands for account. This is a record of all transactions associated with a particular customer or business. Together, these three abbreviations provide a comprehensive overview of the billing process.