A formal or fancy word for indispensable, requisite is often now used in a humorous context. During their princess phase, three year old girls are loathe to leave the house without the requisite beads, tiaras and high heels.
To remember this word, notice how similar it is to required. College students looking to sign up for classes they find interesting often bump up against the word prerequisite, which refers to a list of classes they will need to take before the one they're interested in.
