The "AC resistance" of a trace. For high-speed buses, this must be controlled (e.g. 90 Ω diff pair for USB).
How it looks
Drawing from our real numbers (net class MIPI_100R and the board stackup)
Impedance is not a part you can photograph; it is a property of a track's shape and what is around it. This is the real geometry of a camera (MIPI) pair on CORE3588: track width, gap, height above the ground plane and the board material.
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
Where it applies: the camera pairs in the middle of CORE3588 (amber), kept at constant width and gap along their whole run so the impedance does not change.
How it works
Where it fits
Impedance is why pairs have fixed widths and gaps in their net class, why the stackup must match the fab's real materials, and why the ground plane must stay unbroken under them.