It's the promise of the Father: And behold, I am sending the promise of my Father upon you. But stay in the city until you are clothed with power from on high.” [Luke 24:49, Acts 1:4]
It's what Jesus poured out on the day of Pentecost:
Being therefore exalted at the right hand of God, and having received from the Father the promise of the Holy Spirit, he has poured out this that you yourselves are seeing and hearing. [Acts 2:33] the evidence of the new birth, the gift of holy spirit
It's what we receive when we repent and are baptized: And Peter said to them, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. [Acts 2:38]
It's the gift that Simon thought he could purchase: But Peter said to him, “May your silver perish with you, because you thought you could obtain the gift of God with money! [Acts 8:20]
It's the same thing the Gentiles received when they believed the gospel Peter proclaimed: And the believers from among the circumcised who had come with Peter were amazed, because the gift of the Holy Spirit was poured out even on the Gentiles. [Acts 10:45]
The same gift the apostles received on the day of Pentecost:
If then God gave the same gift to them as he gave to us when we believed in the Lord Jesus Christ, who was I that I could stand in God's way?” [Acts 11:17]
Every Christian is baptized in the gift of holy spirit, and that happens when the person is born again. We were all baptized in spirit, and made to drink of it ..... For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit. [1 Cor. 12:13] .....If anyone thirsts, let him come to me and drink. Whoever believes in me, as the Scripture has said, ‘Out of his heart will flow rivers of living water.’” Now this he said about the Spirit, whom those who believed in him were to receive, for as yet the Spirit had not been given, because Jesus was not yet glorified. [John 7:39]