unofficial era name; private era name
Historical term for era names created and used by commoners or local regions, not by the imperial court. Often contrasted with official era names (元号).
In the Edo period, private era names were sometimes used among commoners separately from the official era names set by the imperial court.
This old document records an unfamiliar unofficial era name.
Compound of 私 (private, unofficial) and 年号 (era name). The term itself is a modern historical classification.