What is Regnal Year?

Legal Definition
A regnal year is a year of the reign of a sovereign, from the Latin regnum meaning kingdom, rule.

The oldest dating systems were in regnal years, and considered the date as an ordinal, not a cardinal number. For example, a monarch could have a first year of rule, a second year of rule, a third, and so on, but not a zero year of rule, which would be nonsensical. Applying this ancient epoch system to modern calculations of time, which include zero, is what led to the debate over when the third millennium began.

Regnal years are "finite era names", contrary to "infinite era names" such as Christian era, Jimmu era, Juche era, and so on.
-- Wikipedia
Legal Definition
Statutes of the British parliament are usually cited by the name and year of the sovereign in whose reign they were enacted, and the successive years of the reign of any king or queen are denominated the "regnal years."
-- Black's Law Dictionary