Used about fats and oils, `unsaturated` marks the kind that mostly comes from plants and fish rather than from land animals — olive oil, peanut oil, avocado, nuts, salmon. These fats are usually liquid at room temperature, and doctors generally treat them as the better ones for your heart. Saturated fats, the other kind, come mostly from meat, butter and coconut, and are usually solid when cold.

The word comes out of chemistry, where 'saturated' means every space is already filled — a saturated fat molecule holds all the hydrogen it possibly can. Unsaturated means there is a gap, a place where the chain is not full. You do not need the chemistry to use the word, but it explains why the same word turns up on a nutrition label, in a science class, and in talk about soaking things in liquid.

On food packaging you will meet the family: unsaturated, monounsaturated (one gap), polyunsaturated (many gaps). Taiwanese labels translate these directly, so once you match the English to the Chinese you will recognise them everywhere.

A gentle honesty note: 'unsaturated = healthy' is the standard advice, and it is roughly right, but nutrition science keeps arguing about the details. Say 'usually considered better for you' rather than treating it as settled fact.