a leafy bough: one of the limbs a tree or vine spreads out, the green growth that makes its crown
This is not the everyday Hebrew word for a branch. It picks out the leafy limbs that spread from a trunk and give a tree its shape, its shade, and its grandeur, and it belongs to a small family of words about finery: the verb פָּאַר means to beautify or adorn, and פְּאֵר is a fine headdress. A tree's boughs, on that picture, are the crown it wears.
Six of the seven occurrences fall inside two of Ezekiel's allegories. In Ezekiel 31 a cedar towers over every other tree: its boughs grow long on plentiful water, birds nest among its forks, wild animals give birth in its shade, and then foreigners fell it and its boughs lie snapped across every ravine in the land. In Ezekiel 17:6 the same word describes something far humbler, the growth a low, sprawling vine sends out. Ezekiel is happy to stack several words for branches in a single line, and this one keeps company with the word for the forking boughs (סְעַפָּה) and the word for the trailing ones (דָּלִיּוֹת).
Isaiah 10:33 uses the singular for the whole leafy top at once, which is why English versions there split between "the bough" and "the boughs." The scene matches Ezekiel's, and so does the point: what grows tall and looks magnificent is exactly what gets lopped off.