Estoppel is a legal doctrine that prevents a person from asserting a claim or right that contradicts what they have previously stated or agreed to, or from denying a fact that has already been established as true. The doctrine is based on principles of fairness, justice, and preventing injustice that could arise if a person is allowed to go back on their word or conduct.

There are several types of estoppel, each serving different purposes:

1. **Equitable Estoppel:**
– Equitable estoppel, also known as estoppel in pais or estoppel by conduct, arises when a person’s conduct leads another person to reasonably rely on certain facts, and it would be unfair or unjust to allow the first person to later deny those facts. Equitable estoppel is often invoked in cases where a person makes a promise or representation, and another person relies on that promise to their detriment.

2. **Promissory Estoppel:**
– Promissory estoppel is a subset of equitable estoppel that specifically deals with promises. It occurs when one party makes a clear and definite promise, and the other party relies on that promise to their detriment. Even if the promise is not supported by a formal contract, promissory estoppel may prevent the promisor from breaking their promise.

3. **Collateral Estoppel (Issue Estoppel):**
– Collateral estoppel, or issue estoppel, prevents a party from relitigating an issue that has already been conclusively determined in a previous legal proceeding. If an issue was central to a prior case, and a court made a final determination on that issue, the same party is generally estopped from raising the same issue in a subsequent case.

4. **Judicial Estoppel:**
– Judicial estoppel prevents a party from taking inconsistent positions in different legal proceedings. If a party adopts a certain position in one case and succeeds, they are estopped from taking a contrary position in a later case if it would undermine the court’s trust in the integrity of the legal process.

Estoppel is often raised as a defense in legal proceedings to prevent a party from changing their position to the detriment of another party. The application of estoppel depends on the specific facts and circumstances of each case, and the doctrine is designed to promote fairness, prevent fraud, and maintain the integrity of the legal system.