James K.A. Smith, in Thinking in Tongues, defines "pentecostal" this way. "By 'pentecostal' I mean to refer not to a classical or denominational definition, but rather to an understanding of the Christian faith that is radically open to the continued operations of the Spirit." (Thanks Chris B. for the question.)
Smith's entire book develops this.
For me this is a killer of a book. As I read it I feel like I'm in a room where doors and windows to the Spirit's activity are opening and fresh winds are blowing in.
