Mounce Concise Greek-English Dictionary

to be, to exist, Jn. 1:1; 17:5; Mt. 6:30; Lk. 4:25, freq.; ἐστί[ν], it is possible, proper, Heb. 9:5; a simple linking verb (“copula”) to the subject and predicate, and therefore in itself affecting the force of the sentence only by its tense, mood, etc., Jn. 1:1; 15:1, freq.; it also forms a frequent circumlocution with the participles of the present and perfect of other verbs, Mt. 19:22; Mk. 2:6

Source: Mounce Concise Greek-English Dictionary