A necessity is something you absolutely need, something essential for survival or for accomplishing an important goal. Food, water, shelter, and clothing are necessities because you can't live without them. A winter coat in Minnesota is a necessity because you need it to stay warm and healthy in freezing temperatures.

When your parents say “we can only buy necessities this month,” they mean the essential things your family truly needs, not the extras or luxuries. A bicycle might feel important, but it's probably not a necessity if you can walk or take the bus to school.

People sometimes say “necessity is the mother of invention,” meaning that when people desperately need something, they become creative and figure out solutions. During the Apollo 13 space mission, when the crew's air filters failed, engineers on Earth had to invent a quick fix using only materials the astronauts had onboard. That necessity drove amazing problem-solving under pressure.

You can also use necessity to describe something that must happen: “Studying became a necessity when she realized the test was harder than she expected.” The word carries a sense of urgency and importance, separating what's truly required from what would simply be nice to have.