The word as used in Pentecostal-Holiness circles means: One who follows the teaching and practices of the Apostles of Jesus Christ. Since the Lord Jesus Christ personally taught these men through His words and life, we believe they knew what the Lord expected of those who desired to commit their life to God through Him.
The Book of Acts is the only New Testament book that describes what the Apostles collectively taught and practiced. Among other things, we find that they spoke in tongues (a language gifted to a person by God without them having to learn the language by the process of human effort), and they prayed for other people to receive this wonderful gift of supernatural communication.
They also baptized repentant persons by having them fully immersed in water to represent burial. Since a person who turns to God must first consider him/ herself "dead" to their old life, water baptism buries the "old" sin nature with Christ. The person is "resurrected" from the water to live for God through the "new" spiritual nature.
When baptizing a repentant person, the Apostles in every example given to us in the Book of Acts always use a phrase that contained Jesus' Name. The words "Father," and "Holy Spirit" do not appear in reference to baptism since God as "the Father" and "Holy Spirit" did not die, was not buried, and therefore did not raise from the dead.
Instead, we find that since we are redeemed from sin unto God by the death, burial and resurrection of the Lord Jesus Christ, the Apostles understood that we too must die with Christ (through repentance), be buried with Him (through water baptism having His Name invoked over us), and raise from this watery grave to live a new life (through the incarnation of His Spirit in the believer).
Finally, we believe that there is only One God - numerically! "Father," "Son" and "Holy Spirit" are terms that describe the various aspects of this One God's RELATIONSHIP to them that "believe on His Name" (John 1:12).