Final answer:
Failure to disclose material fact is defined as concealment, which refers to the failure to reveal relevant information in the context of agreements or contracts.
Step-by-step explanation:
Failure to disclose material fact can be defined as concealment. Concealment in legal terms refers to the failure to reveal information that one knows to be relevant to a contract or an agreement. This action could potentially impair the obligation of contracts as it may abridge the privileges or immunities of citizens and can be considered an act that denies the due process and equal protection of the laws.
While the term 'fraud' is related to intentional deception to secure unfair or unlawful gain, 'waiver' refers to the voluntary relinquishment of a known right, and 'estoppel' is a legal principle that prevents someone from arguing something contrary to a claim previously made or an act previously performed. Therefore, the correct answer to whether failure to disclose material fact is concealment, fraud, waiver, or estoppel is concealment.