The difference between Invalid and Valid

When used as adjectives, invalid means not valid, whereas valid means well grounded or justifiable, pertinent.

Invalid is also noun with the meaning: any person with a disability or illness.

Invalid is also verb with the meaning: to exempt from duty because of injury or ill health.

check bellow for the other definitions of Invalid and Valid

Invalid as an adjective:

Not valid; not true, correct, acceptable or appropriate.

Examples:

"Your argument is invalid because it uses circular reasoning."

"This invalid contract cannot be legally enforced."

Invalid as a noun (dated, sometimes, offensive):

Any person with a disability or illness.
Invalid as a noun (dated, sometimes, offensive):

A person who is confined to home or bed because of illness, disability or injury; one who is too sick or weak to care for themselves.
Invalid as a noun (archaic):

A disabled member of the armed forces; one unfit for active duty due to injury.

Invalid as a verb (British, transitive):

To exempt from duty because of injury or ill health.

Examples:

"He was invalided home after the car crash."
Invalid as a verb (transitive):

To make invalid or affect with disease.

Valid as an adjective:

Well grounded or justifiable, pertinent.

Examples:

"I will believe him as soon as he offers a valid answer."
Valid as an adjective:

Acceptable, proper or correct.

Examples:

"A valid format for the date is MM/DD/YY."

"Do not drive without a valid license."
Valid as an adjective:

Related to the current topic, or presented within context, relevant.
Valid as an adjective (logic):

Of a formula or system: such that it evaluates to true regardless of the input values.
Valid as an adjective (logic):

Of an argument: whose conclusion is always true whenever its premises are true.

Examples:

"An argument is [[valid]] if and only if the set consisting of both (1) all of its premises and (2) the contradictory of its conclusion is inconsistent."