1. Correlative to or, and preceding it.

2. One or other of any number; one of two; each; every one, separately considered.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• aithor,
• eitht,
• nneither,
• eithin,
• beirther,
• niether,
• ithere,
• etier,
• eiter,
• ethen,
• eithers,
• afther,
• oithers,
• fiuther,
• eaither,
• ohther,
• otther,
• etherm,
• aother,
• raither

• eater,
• other,
• weather,
• anther,
• neither,
• zither,
• leather,
• wither,
• heather,
• ether,
• feather,
• tether,
• eider.

The adjective form of the word "either" is "either".

The plural form of the word "either" is "neither"

The singular form of the word "either" is "either". The word "either" does not change whether it is used in a singular or plural form. It is a pronoun and an adverb that can be used to refer to one of two options, regardless of the number being referred to.