Coherence
Also called Coherent light, Coherent sources
Coherence is a fixed, unchanging phase relationship between wave sources. Coherent sources make an interference pattern that holds still long enough to see, which is why slit experiments use one laser rather than two separate lamps.
Two slits lit by a single laser stay locked in step. Two separate bulbs do not, and that difference is the whole idea.
Interference needs a stable phase relationship, not merely two waves. An ordinary lamp emits in short bursts whose phases jump at random, so whatever bright and dark bands it would form rearrange themselves far too fast to see and average out to plain illumination. Coherent sources hold their phase relationship fixed, so every point on the screen keeps the same path length difference and therefore the same brightness.
How much of this is AP. The AP Physics 2 CED names coherence once, in EK 13.1.A.1.iii: a laser is a common source of a single coherent, monochromatic beam of light that can be modeled as a ray. It gives the word no definition, no symbol and no equation, and the Unit 14 interference topics do not use it. Expect coherence as a condition handed to you in the stem of a question, not as something you calculate.
Coherent and monochromatic are different words. Monochromatic means one wavelength. Coherent means a locked phase relationship. The CED lists both for a laser because a laser happens to be both. Two independent lasers of identical wavelength are monochromatic and still incoherent with each other.
That is also why the double-slit setup uses one source and two slits rather than two sources. Splitting a single wavefront guarantees the slits stay in step, so the bright fringes sit still instead of smearing.