Learner's definition of NECESSITY
1 [count] : 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]
2 [noncount], formal : 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]
by necessityorout 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