What does "is" mean?

Is: 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.