noun
usual diet; habitual food
Refers to the food one normally eats, as opposed to a special or restricted diet. Often used in contrast with 精進料理 or 病食.
彼は病中も素食を変えなかった。
He didn't change his usual diet even while sick.
noun
vegetarian food; vegetarian diet
Specifically a diet that excludes meat and fish. Can refer to the food itself or the dietary practice.
彼女は健康のために素食を始めた。
She started a vegetarian diet for her health.
素 (plain, usual) + 食 (food, diet). The 'vegetarian' sense likely developed from the idea of a simple, plain diet without meat.