An interjection is a part of speech that demonstrates the emotion or feeling of the author. These words or phrases can stand alone, or be placed before or after a sentence.
Selah: is defined as a Hebrew word that has been found at the ending of verses in Psalms and has been interpreted as an instruction calling for a break in the singing of the Psalm or it may mean “forever.
Hallelujah vs. Alleluia: Both words mean the same thing, the difference lies in the language. They are from the Hebrew word which means Salvation, glory and power belong to our God or Glory be to God/YHWH! Hallelujah is used 4 times in the NT, all of which are in Revelation 19:1-6. Additionally, Hallelujah is used 24 times in the Old Testament, all of which are in 15 of the Psalms between Psalm 104-150.
Hosanna: The word hosanna derives from a Hebrew word and is used as two completely different uses. One is to showing gratitude, praise, or joy or simply thanking him. The other use is for pleading for help.
Amen: is derived from the Hebrew āmēn, which means we have heard what was said in the prayer and we express our “certainty,” “truth,” and “agreement”. It is used in Jewish, Christian and Muslim worship as a concluding word or response during or after prayer.