"Baptism in the Holy Spirit"? If you have been involved in the Charismatic Renewal, you immediately know what I this term refers to. If you have not, you may not be as clear. In short, it refers to a tangible outpouring of the Holy Spirit upon someone when prayed over by others to receive this special grace. It is often accompanies by a deepening of faith and zeal for the faith. It is the backbone of the Charismatic Renewal. Just to be clear; it is NOT a sacrament and is not referring to sacramental baptism.
The Pontifical Council for the Laity recently got together to study and discuss "baptism in the Holy Spirit." Here is a link to the Zenit article summarizing the meeting.