is Gr. ἴσοσ equal.] A prefix or combining form, indicating identity, or equality; the same numerical value; as in isopod, isomorphous, isochromatic. Specif.:

(a) (Chem.) Applied to certain compounds having the same composition but different properties; as in isocyanic.

(b) (Organic Chem.) Applied to compounds of certain isomeric series in whose structure one carbon atom, at least, is connected with three other carbon atoms; – contrasted with neo- and normal; as in isoparaffine; isopentane. Is, intransitive verb [AS. is; akin to G. & Goth. ist, L. est, Gr. Skr. asti. √9. Cf.

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.

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

For thy is I come, and eke Alain. Chaucer.

Aye is thou merry. Chaucer.

☞ 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.

To-morrow is the new moon. 1 Sam. xx. 5.

-- Webster's unabridged 1913