compassionate: moved with tender, parent-like mercy toward someone in need
This is not a general word for being nice or kind. In every one of its appearances it describes a deep, gut-level compassion, the kind a parent feels for a child, and almost always it is God who is being described this way. The word shares its root with the Hebrew word for a mother's womb, and many readers hear that connection in it: a mercy that is bodily and instinctive rather than calculated, the ache of love a mother feels for the child she carried.
The word rarely stands alone. Eleven of its thirteen appearances place it inside a single famous sentence about God's character that begins when God announces his own name to Moses: "the LORD, the LORD, a God compassionate and gracious, slow to anger, and abounding in steadfast love and faithfulness" (Exodus 34:6). That line became something close to a creed in Israel. Later writers quote it, shorten it, and even argue with it: the psalmists lean on it for comfort, Joel uses it to call people back to God, and Jonah throws it back at God in frustration that God is too merciful to destroy Israel's enemies.
Paired with it almost every time is a second word, "gracious," and the two together cover two sides of God's kindness. "Compassionate" points to the warm, parental tenderness God feels; "gracious" points to favor freely given by someone with the power to withhold it. Read together, they say God is both the affectionate parent and the generous king.
Because the word is reserved for this rich, mostly divine use, translators reach for several different English words for it, and no single one captures the whole. "Merciful," "compassionate," and "tender" each catch part of it; the bodily, womb-deep warmth is the part English most easily loses.