Coherence (Optics)
Definition
How 'in step' a light wave is — laser light is highly coherent, which is why it paints crisp interference patterns.
Technical definition
Temporal coherence (phase correlation over time, coherence length L_c = c/Δν) and spatial coherence (across the beam). Interference requires coherence.
Example
A laser pointer makes speckly dots because its long coherence length lets scattered waves interfere; an LED's short coherence washes patterns out.
Common confusion
Coherence is not brightness. A dim laser interferes beautifully while a brilliant arc lamp barely interferes at all — what matters is phase orderliness, not power.