What is the plural of series?

The plural of series is series.

Series is an invariant noun: the singular and plural are identical.

All forms of series

as a noun

base: series
plural: series

Series in a sentence

After a series of economic downturns, consumer confidence is at an all-time low.OjisanSeiuchi
The shell fell into the ammunition depot, which provoked a series of explosions.carlosalberto
As part of the project, a series of round-table discussions will take place.Nylez