Refers to the act or state of boiling, often used in compounds like 沸き上がる (to boil up) or 沸き立つ (to seethe). As a standalone noun, it is less common than the verb 沸く.
The kettle is slow to boil.
I read the newspaper while waiting for the water to boil.
沸き上がる湯気を見つめる。
I gaze at the steam rising from the boiling water.
This kettle is slow to boil the water.
沸く is the intransitive verb meaning 'to boil', while 沸き is its derived noun form referring to the boiling itself, and learners often mix up which form to use in a sentence.
The noun form of the verb 沸く (to boil), derived from the continuative stem (連用形). The kanji 沸 is used for boiling water or emotions.