"It seems to me that the term is a broad, overarching one that includes the whole great saving, sanctifying, and empowering work of the Spirit in this age. I don’t think it is a technical term that refers to one part of the Christian life—say conversion, or speaking in tongues, or a bold act of witness. It is the continual, and sometimes extraordinary, outpouring of the Holy Spirit on God’s people. It immerses them not just in one or two, but in hundreds of his powerful influences.
In other words, if you are not born again, one way to describe your need is that you need to be baptized with the Spirit. That is, you need to be plunged into God’s Spirit with the effect that you will be born again and come to faith in Christ. If you are born again, but you are languishing in a season of weakness and fear and defeat, one way to describe what you need is to be baptized in the Spirit. That is, you need a fresh outpouring of his Christ-revealing, heart-awakening, sin-defeating, boldness-producing power. Every spiritual need that we have before and after conversion is supplied by Christ immersing us in greater and lesser degrees in the Holy Spirit".
John Piper - "This Is He Who Baptizes with the Holy Spirit" - Bethlehem Baptist Church, Illinois - November 23, 2008