song: a composed musical work or poem, and the act of performing it in worship
The word covers the full range from a specific composed piece to the living practice of singing it. In the oldest stratum it names the great narrative odes, the Song of the Sea, the Song of Moses, Deborah's song, long, formally crafted poems that celebrated decisive acts of God. The same noun then travels across the whole psalm superscription tradition, where it labels a poem as a finished artistic work ready for public performance.
In the Chronicles literature a distinct institutional weight gathers around the word. The Levitical singers organized by David and Solomon are described as ministering with "songs of the LORD," and the Chronicles writers use the noun to name the cultic-musical office itself, not just what the singers sing but what they do as an ordained station in the sanctuary. This is the word's most concentrated site of use in the Hebrew Bible.
The two grammatical forms, masculine שִׁיר and feminine שִׁירָה, carry the same meaning; the feminine tends to appear in the older, longer narrative poems (the Song of the Sea, the Song of Moses), possibly as a marker of poetic genre. The standard lexica (HALOT, BDB, DCH) treat both under a single entry without separating them as homonyms.
The phrase שִׁיר הַשִּׁירִים (Song of Songs, literally "the song of songs") is a Hebrew superlative, singling out one composition as the finest of all songs. The prophets invoke the word to sharpen judgment: when God silences "your songs" (Amos 5:23) or a city's songs become lamentation (Amos 8:10), the removal of music signals the removal of divine blessing.