a psalm: a song composed to be sung with instrumental music
This word is the technical name for a particular kind of sacred song, one meant to be sung to the plucking of strings or other instruments. It appears almost only at the very top of a psalm, in the heading that tells the reader (and the worshiping community) what sort of piece follows and, often, who composed it and how it was to be performed.
It is built from a verb that means to make music, especially by plucking an instrument, so the word carries the idea of a melody that is played as much as sung. That is what sets it apart from a plain "song": a psalm in this sense is words wedded to music, an arranged composition for worship rather than a poem simply recited.
Because it is used so consistently and in one setting, the word does not branch into different senses. Wherever it occurs it names the same thing, a worship song set to music, which is why English Bibles render it with the single steady word "psalm."