1. To be obliged or required by morality, law, or custom: Citizens must register in order to vote.
2. To be compelled, as by a physical necessity or requirement: Plants must have oxygen in order to live.
3. Used to express a command or admonition: You must not go there alone. You simply must be careful.
4. To be determined to; have as a fixed resolve: If you must leave, do it quietly.
5.
a. Used to indicate inevitability or certainty: We all must die.
b. Used to indicate logical probability or presumptive certainty: If the lights were on, they must have been at home.