The term of a patent is the maximum time during which it can be maintained in force. It can vary based on the type of patent and the jurisdiction, but it is usually expressed in a number of years either starting from the date of the patent application or from the date of grant of the patent. Almost all patents last 20 years if kept in force.
In most jurisdictions, patent annuities (renewals) or maintenance fees have to be regularly paid in order to keep the patent in force. We consider a patent expired when it either reaches its maximum term time or the grant holder does not pay the renewal fee in due time.
Reinstating an expired patent is an extensive and costly procedure with no guarantee of success.