a leaf: the green growth of a tree or plant, and by extension its foliage
The word names a leaf, the flat green growth of a tree or plant, and by extension the foliage that covers it. It is an everyday, physical word: the fig leaves Adam and Eve sewed together (Genesis 3:7), the olive leaf the dove carried back to Noah (Genesis 8:11), the leafy boughs Israel gathered to build shelters at the Feast of Booths (Nehemiah 8:15).
From that plain picture the poets and prophets built one of the Bible's most vivid images of human life. A leaf that stays green and never withers stands for a flourishing, well-rooted life: the person who delights in God is like a tree planted by streams of water whose leaf does not wither (Psalm 1:3, echoed in Jeremiah 17:8), the righteous "flourish like a green leaf" (Proverbs 11:28), and the trees along the river flowing from the temple keep leaves that never fade and even bring healing (Ezekiel 47:12). The New Testament takes up that last picture with its own word for leaf (φύλλον) in the tree of life whose leaves heal the nations.
The same leaf, dried and blown about, becomes a picture of frailty and coming judgment. "We all fade like a leaf" (Isaiah 64:6); a wind-driven leaf is so slight that God need not bother to chase it down (Job 13:25), yet the mere rustle of one can send the guilty running (Leviticus 26:36). When a nation falls under judgment its people wither like an oak whose leaf fades (Isaiah 1:30; compare Isaiah 34:4 and Jeremiah 8:13).
The same fading imagery clings to the blossom (צִיץ), which blooms brightly and quickly falls, while the branch (עָנָף) is the woody limb that carries the foliage rather than the leaf itself.