glad: the settled state of being joyful, describing a person, a heart, or, more rarely, a sound or a setting as cheerful and pleased
This word is the adjective built on the same root as the verb שׂמח, "to rejoice," and the noun שׂמחה, "joy." Where the verb names the act of becoming glad and the noun names the joy itself, this word simply describes someone, or something, as already in that glad state. It usually describes a person outright, or the heart standing in for the person, the way Proverbs calls a heart cheerful. Less often it stretches further, to a sound or a setting, as when Isaiah calls the noise of a city joyful. Curiously, the Bible almost never puts this adjective and its sister noun "joy" in the same verse. Whichever one is used, it carries the whole sentence on its own.
Because this root is Hebrew's ordinary, all-purpose word for gladness rather than a specialized poetic or cultic term, this adjective turns up everywhere gladness does: it fills the great pilgrim feasts, where Israel is told to be nothing but joyful before God, and it rings through a crowd so loud the ground seems to shake at a king's coronation. Just as easily it settles into one quiet person, as when Proverbs says a cheerful heart does the body good like medicine. The same plain word covers a nation's loudest festival and a single person's quiet contentment.
The word itself carries no verdict about whether the gladness is good or bad. What decides that is the thing a person is glad about. When the object is a feast, a rescue, or a child, the gladness is healthy and celebrated. But the same word turns ugly when someone is glad at a neighbor's ruin, glad to do evil, or glad over a hollow, worthless victory. The joy is equally real either way; only its target changes.