Final answer:
The word 'invalid' can mean either a person weakened by illness or injury when used as a noun or something that is not valid or acceptable when used as an adjective.
Step-by-step explanation:
The word 'invalid' can have two distinct meanings depending on the context in which it is used. If used as a noun, it refers to a person made weak or disabled by illness or injury. Alternatively, when used as an adjective, it means something that is not valid, not correct, or not acceptable due to a lack of legal force or because it does not adhere to the necessary rules or norms.
It's important to discern which meaning of 'invalid' is applicable based on the context of the sentence. For example, 'The contract was declared invalid because it was missing a signature,' uses the adjective form, whereas 'After the accident, he became an invalid needing constant care,' uses the noun form.