Iso- \I"so-\, Is- \Is-\ [Gr. 'i`sos 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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Is- \Is-\ See Iso-.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Is \Is\, v. i. [AS. is; akin to G. & Goth. ist, L. est, Gr. ?, Skr. asti. [root]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.

Note: 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.

Note: 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.

Source: Webster's Revised Unabridged Dictionary (1913)