Boyd’s Bible Dictionary:

(praise-song). Spiritual song (Matt. 26:30; Acts 16:25; Eph. 5:19; Col. 3:16).

Concise Bible Dictionary:

These occur in this order: “psalms, hymns, and spiritual songs” (Eph. 5:19; Col. 3:16). The word “psalms” is the same as the Greek ψαλμοῖς; so the word “hymns,” ὔμνοις; for “spiritual songs” it is ὠδαῖς πνευματικαῖς, spiritual odes or songs. There can be no doubt that the Psalm of David had been used by the devout of Israel as songs of praise, and some of these may have been used in the early church, such as Psalm 23; Psalm 103, which in substance have found their way into nearly all modern hymnals.
But the new dispensation required new songs of praise, and some may have been written that retained the name of psalms; others were called hymns, which apply to those compositions which are addressed to the Father or the Son, or directly to God. The word used for “songs” is employed in Revelation 5:9 for the song of the redeemed; and in Revelation 14:3 for the new song; and in Revelation 15:3 for the song of Moses, the sentiments of which are often repeated in the Psalms (compare also Ex. 15:1-19; Deut. 32:1-44).
It will be noticed that the passages in Ephesians and Colossians do not refer to singing in the assembly, and the one in Colossians may be punctuated thus: “in all wisdom teaching and admonishing one another, in psalms and hymns, and spiritual songs, singing with grace in your hearts to the Lord.” The Lord sang a hymn with His disciples at the last passover; and we may be sure that there was singing in the assemblies. The heathen Pliny bore testimony that the Christians sang “odes to Christ as God.” Christ is twice represented as praising God, in the midst of His congregations, that is, in the assembly, and in the great congregation of Israel and those associated with them (Psa. 22:22,25). Among hymns generally it is easy to see that some are hymns of praise; others recount what God has done; others speak of what Christ has suffered; others refer to future blessing; and again others are really prayers.