Lipid
Definition
Fats, oils, and membranes: energy-dense molecules that do not mix with water, built mostly from long hydrocarbon chains.
Technical definition
Lipids are hydrophobic biomolecules including triglycerides (energy storage), phospholipids (membrane bilayers), and steroids (signaling). They store about 9 kcal/g, more than double carbohydrates, and form the structural basis of cell membranes.
Example
Butter (saturated fat, solid) and olive oil (unsaturated fat, liquid) are both triglycerides: the kinked chains of unsaturated fats cannot pack tightly.
Common confusion
Not all lipids are fats you eat. Phospholipids build every cell membrane and steroids include hormones like testosterone: lipid is a solubility category, not a food group.