How should you describe to someone what lactose intolerance is?

Lactose deficiency: Lactose intolerance = lactase deficiency. Lactase is the enzyme that digests lactose. The more severe your deficiency the less milk it will take to give you symptoms.

The basics: Lactose intolerance is basically a transient or permanent loss of the ability to digest/absorb lactose sugar. The membrane in your gut lacks an enzyme needed, so if you consume lactose, it passes through without absorption allowing gut bacteria to feed on it & produce gas and acid. This makes you gassy & cramp. Avoid ingesting lactose and you avoid symptoms.