The baptism of the Holy Spirit is the once-for-all act of the exalted Christ in pouring out the Spirit to create and publicly establish His New Covenant church, incorporating Jews, Samaritans, and Gentiles into one body.
Pentecost is a redemptive-historical event, not a repeated event in every believer’s experience. Christ poured out the Spirit once to constitute His church. Acts records the public expansion of that one church: Jews at Pentecost, Samaritans in Acts 8, and Gentiles in Acts 10.
This must not be confused with regeneration or indwelling. Every believer is given life by the Spirit and is indwelt by the Spirit, but Spirit baptism names Christ’s historical act of forming and publicly establishing His New Covenant body.
See in Scripture: Matthew 3:11; Acts 1:4–8; Acts 2:1–47; Acts 8:4–25; Acts 10:1–48; Acts 11:1–18; 1 Corinthians 12:12–13