invalid

Something that is invalid does not count because it is not acceptable, not legal, or not correct. You might see this with an expired passport, a broken ticket, or an argument that falls apart when someone checks the facts. In each case, the thing cannot be used or trusted in the usual way. The word also has an older noun use for a person made weak by illness, though that meaning is less common in modern everyday English. The pronunciation changes with the meaning: in-VAL-id usually means "not valid," while IN-vuh-lid is the older noun.

The word invalid covers several related meanings, all centered on the idea that something cannot be accepted, used, or trusted in the normal way. At its core, it means "not valid" because it lacks the right status, support, or correctness.

You might hear about an invalid passport if it is expired or damaged, which means it cannot be used for travel. A legal contract can also be invalid if it does not meet the required conditions, so a court may not accept it. In logic, an argument is invalid if the reasoning does not properly support the conclusion. In science, test results may be called invalid if something went wrong in the process, such as contamination or a mistake in measurement.

Historically, the noun form of the word often described a person made weak by illness or injury, especially someone unable to live or work as usual. You may still see this meaning in older books or historical writing, but it is less common in modern daily speech and can sound dated today.

Related forms such as invalidate and invalidated are also common. For example, someone may say their feelings were invalidated when others dismissed their experience. That idea is connected, but it is slightly different from the main adjective. Whether it describes a document, an argument, a scientific result, or in older usage a person's condition, invalid points to something that lacks the qualities needed to be accepted as official, sound, or effective.