Linear mass density

Mass spread along a line. If a 0.60 kg cord is 2.0 m long and uniform, its linear mass density is 0.30 kg/m.

AP Physics C: Mechanics uses it for nonuniform objects. EK 2.1.B.3.i defines the linear mass density of a rod or other linear rigid body as the derivative of the rod's mass with respect to the position of the differential mass element on the rigid body, and both Physics C sheets print $\lambda = \frac{d}{d\ell}m(\ell)$ with a symbol list reading lambda equals linear mass density. Turn it around for $dm = \lambda\, d\ell$, the piece that goes inside the center of mass and rotational inertia integrals.

AP Physics 2 needs the quantity and never names it. The speed of a wave on a string appears on that equation sheet as $v_{\text{string}} = \sqrt{F_T/(m/\ell)}$, with mass over length spelled out rather than compressed into a symbol, and the phrase linear mass density does not appear in the AP Physics 2 framework. So on that exam you calculate it and call it mass per unit length.

The reading it explains: a thicker string under the same tension carries a slower wave, which is why the low strings on a guitar are the heavy ones.

The letter is reused. The Physics C symbol lists give $\lambda$ as linear mass density, while the AP Physics 2 sheet lists it as wavelength or decay constant. Read the symbol list on the sheet in front of you rather than the one you remember.