Answer:
Here is an excerpt from a previous Answers.com topic regarding estoppel. I take no credit for this definition, but believe it will accurately address your inquiry.
A doctrine of law that stops one from later denying facts which that person once acknowledged were true and others accepted on Good Faith.
Example: Abel signs a certificate acknowledging that he owes $10,000 on a Mortgage as of a certain date. Later he contends that he owed only $5,000. Abel is prevented from asserting this new contention under estoppel.