after retirement; after resigning
Refers to the period or state following leaving a job, whether by retirement or resignation. Often used as a temporal noun or adverbially.
I'm looking forward to life after retirement.
After retiring, he moved to the countryside.
定年後 means 'after reaching mandatory retirement age,' while 退職後 is broader and includes voluntary resignation at any age.
Compound of 退職 (retirement/resignation) + 後 (after). The reading is the standard on'yomi combination.