Learner's definition of NECESSITY
something that you must have or do
something that is necessary
Sunscreen is an absolute necessity for the beach.
food, clothes, and other basic necessities
Getting plenty of rest is a necessity.
Without a car, living close to work is a necessity.
All we took with us on our hiking trip were the bare necessities.
Many families cannot even afford the basic/bare necessities of life. [=things that a person must have in order to survive]
the quality of being necessary
— usually + of or for
She talked about the necessity of having the right training. [=she said that having the right training is necessary]
He questioned the necessity for the change. [=he questioned whether the change was necessary]
out of necessity
because of conditions that cannot be changed
make a virtue (out) of necessity
necessity is the mother of invention
— used to say that something must happen or must be the way it is