"to introduce an explanation is typical Johannine style; it was used before in 1:19, 3:19, and 15:12. The Evangelist here defines 'eternal life' for the readers. It is not just unending life in the sense of prolonged duration. Rather it is a quality of life, with its qualitativeness derived from a relationship with God. Having eternal life is here defined as being in relationship with the Father, the one true God, and Jesus Christ whom the Father sent. Cristov is not characteristically attached to Jesus' name in the Fourth Gospel; it occurs elsewhere primarily as a title and is used with Jesus' name only in 1:17. But that is connected to its use here: the statement here in 17:3 enables us to correlate the statement made in 1:18 of the Prologue, that Jesus has fully revealed what God is like, with Jesus' statement in 10:10 that he has come that people might have life, and have it abundantly. These two purposes are really one, according to 17:3, because (abundant) eternal life is defined as knowing (being in relationship with) the Father and the Son. The only way to gain this eternal life, that is, to obtain this knowledge of the Father, is through the Son (cf. 14:6). Although some have pointed to the use of ginwvskw here as evidence of Gnostic influence in the Gospel, there is a crucial difference: for John this knowledge is not intellectual, but relational. It involves being in relationship."
While I'm not endorsing his comments as a whole here, a number of commentators and Johannine scholars would agree that Jesus (through John's writing) is defining everlasting/eternal life in this verse. On the other hand, Witnesses also say that John 17:3 stipulates how one might obtain life eternal, namely, by knowing Jehovah and his Son.