A person who is very sick or injured and needs frequent care is an invalid. Also, something no longer current or useful is invalid.
The main uses of this word have to do with people and things that don't work very well anymore. A driver's license that has expired is invalid — it needs to be renewed. If you lost your health insurance, it's invalid. People who are called invalids are disabled or incapacitated severely. Be careful with this word, because some think it is inappropriate, since it sounds as if some people are not as valid as others based on physical capability.