The word “minister” is used in three senses in Scripture:
(1) of the service or ministration of both priests and Levites rendered unto the Lord Exodus 28:35, Exodus 28:43 :
(2) of the ministrations of the Levites as rendered to the priests, to aid them in divine Service Numbers 3:6 :
(3) of any service or ministration, especially one rendered to a man of God, as that of Joshua to Moses Numbers 11:28.
The application of it to Samuel as ministering to the Lord before Eli the priest accords “most exactly” with Samuel’s condition as a Levite.