Bygone and Erstwhile

Bygone adjective - No longer existing.

Erstwhile is a synonym for bygone in time topic. In some cases you can use "Erstwhile" instead the word "Bygone" as an adjective or an adverb, when it comes to topics like past, in the past.

Erstwhile adjective - Having been such at some previous time.

Bygone is a synonym for erstwhile in former topic. You can use "Bygone" instead the word "Erstwhile" as an adjective or an adverb.