The concentration of cholesterol circulating in the bloodstream, carried in lipoprotein particles, a central marker of cardiovascular risk. Total cholesterol matters less than its distribution: high LDL, the bad cholesterol, drives artery-clogging atherosclerosis, while higher HDL is protective, and triglycerides add independent risk. Diet influences the pattern substantially, with saturated and trans fats raising LDL, soluble fiber and unsaturated fats improving it, and excess calories and alcohol raising triglycerides. Guidelines set target levels based on overall risk rather than cholesterol alone. The number on a lab report thus summarizes genetics, diet, activity, and medication in a single panel, making it both a diagnostic tool and a motivator for the lifestyle changes that improve it.