Webster's Revised Unabridged Dictionary · 1913

Is-

Is-

See Iso-.

Is , intransitive verb

[Anglo-Saxon is; akin to German & Gothic ist, Latin est, Greek {not transcribed}, Sanskrit asti. r9. Compare Am, Entity, Essence, Absent.]

The third person singular of the substantive verb be, in the indicative mood, present tense; as, he is; he is a man. See Be.
For thy is I come, and eke Alain. — Chaucer
Aye is thou merry. — Chaucer
To-morrow is the new moon. — 1 Sam. xx. 5

In some varieties of the Northern dialect of Old English, is was used for all persons of the singular.

The idiom of using the present for future events sure to happen is a relic of Old English in which the present and future had the same form; as, this year Christmas is on Friday.