The adjective form of rachamim (compassion/womb). Appears in God's self-declaration in Exodus 34:6 — the foundational statement of God's character that echoes throughout the OT.
Exodus 34:6 — 'The LORD, the LORD, a God merciful [rachum] and gracious' — is the most cited OT text within the OT itself. Psalms and prophets invoke this declaration when calling on God's character.
The 13 attributes of God (Exod 34:6-7) are called the Thirteen Middot in Jewish tradition. Rachum heads the list after the name YHWH itself — compassion is God's leading attribute in self-revelation.