Reserve means to keep something back for later use or to set something aside for a specific purpose or person. When you reserve a table at a restaurant, you're making sure it will be available for you at a specific time. When a library lets you reserve a popular book, they hold it for you so you can check it out when it becomes available. Hotels, theaters, and airlines all use reservation systems to help people plan ahead.

As a noun, reserve can also mean supplies kept on hand for emergencies or future needs. A school might keep a reserve of extra textbooks in case students lose theirs. Your body keeps energy reserves as fat, storing fuel for when you need it later. Countries maintain oil reserves and grain reserves to prepare for shortages. A basketball coach might keep strong players in reserve on the bench, ready to substitute them in when needed.

When someone shows reserve, they're holding back their emotions or opinions, staying calm and controlled rather than expressing everything they feel. A person who speaks with reserve chooses their words carefully and doesn't blurt out every thought. This kind of reserve can show maturity and self-control, though sometimes people mistake it for coldness or unfriendliness. The adjective form is reserved, describing someone who tends to be quiet and private rather than outgoing and expressive.