The word invalid has two completely unrelated meanings:

1. Not valid, true, or legally acceptable. An invalid argument contains faulty reasoning that doesn't hold up under scrutiny. An invalid passport has expired and can't be used for travel. A teacher might mark a test answer invalid if it doesn't follow the instructions. In science, an invalid experiment means the results can't be trusted because something went wrong with the method. The opposite of invalid in this sense is valid, meaning sound, acceptable, or legally binding.
2. A person who is weakened or disabled by illness or injury. In old novels like The Secret Garden, you'll encounter characters described as invalids who spend much of their time in bed or unable to participate in everyday activities. This meaning is pronounced differently (IN-vuh-lid rather than in-VAL-id) and is rarely used today, as most people now prefer more respectful terms like “a person with a disability” or simply describing the specific condition someone has.