'Estoppel' refers generally to the phenomenon of someone being stopped from exercising what they think is a right but which they have surrendered by past behaviour.
In the day-to-day cut & thrust of business the term refers to the situation where one may be held to a position against the beneficial term of a contract if one has acted overtly against it.
In law 'estoppel' includes several similar themes. There are both 'cause of action' and 'issue' estoppel, which prevent specified facts from being argued in court again, and which some judges consider to be aspects of res judicata. Then there is stare decisis, the 'legal' counterpart of estoppel, acting to prevent the same legal issue from being re-litigated.
