ApoB
Also: apolipoprotein B
In plain language
A protein found on the "bad" cholesterol particles that cause artery disease; measuring it counts how many of these harmful particles you have.
Technical definition
The primary structural apolipoprotein of atherogenic lipoproteins (LDL, VLDL, IDL, and Lp(a)); one ApoB molecule per particle, so ApoB concentration reflects the total number of atherogenic particles and is a strong predictor of cardiovascular risk.
Because each atherogenic lipoprotein carries exactly one ApoB, measuring ApoB counts the number of cholesterol-carrying particles rather than the amount of cholesterol they hold. Many lipid specialists regard it as a more accurate marker of cardiovascular risk than LDL cholesterol alone.