The definition of bygone means something that is in the past.
An example of a bygone is an old fight that a couple decide to leave in the past.
that has or have gone by; past; former
anything that is gone or past
let bygones be bygones
to let past offenses or disagreements be forgotten
Gone by; past: bygone days.
One, especially a grievance, that is past: Let bygones be bygones.
- gone by