ungültig
invalid / expired
No longer valid or effective because a deadline or period has expired.
Ihr Reisepass ist seit letztem Monat ungültig.
Your passport has been invalid since last month.

invalid
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invalid / expired
No longer valid or effective because a deadline or period has expired.
Ihr Reisepass ist seit letztem Monat ungültig.
Your passport has been invalid since last month.

The word is made up of the prefix 'un-' and the adjective 'gültig' (valid). The prefix 'un-' negates the meaning of 'gültig' and means 'not'.
Diese Fahrkarte ist leider ungültig.
Unfortunately, this ticket is invalid.
'Ungültig' is often used in the construction 'für ungültig erklären' (to declare invalid). This means that an official authority decides that something is no longer valid.
Die Wahl wurde wegen Betrugs für ungültig erklärt.
The election was declared invalid because of fraud.
Common things that can be 'ungültig' (invalid) are official documents, agreements, or actions: a ticket, a passport, a contract, a vote (in an election), or an attempt (in sports).
Ein Stimmzettel ohne Kreuz ist ungültig.
A ballot paper without a mark is invalid.
valid
'gültig' and 'ungültig' are opposites. 'gültig' means that something officially counts, may be used, or applies to a situation. 'ungültig' means that something does not count, for example because a deadline has passed, a form is missing, or a rule was not followed. This applies to documents, tickets, contracts, and also in sports. Put simply: what is 'gültig' is accepted; what is 'ungültig' is not accepted.

Der Stimmzettel war ungültig, weil zwei Felder angekreuzt waren.
The ballot was invalid because two boxes had been checked.
Die Fahrkarte ist heute noch gültig.
The ticket is still valid today.