What is an acceleration clause in a Promissory Note?

An acceleration clause states that the creditor reserves the right to demand payment in full at any time. The idea behind an acceleration clause is that if the debtor misses a payment, the creditor can demand immediate payment of the entire past due balance. Of course, customers may be reluctant to agree to this particular clause, but it is an essential part of the leverage you need to ensure that the debtor recognizes the importance of making each payment due under the Note as agreed.

Taken from "1001 Collection Tools, Tips and Techniques" by Michael C. Dennis