Longman Dictionary of Contemporary English
Wiktionary
n. (context British Ireland English) A semi-sweet biscuit; dunked in tea, or crushed as the base of a cheesecake
Wikipedia
A digestive biscuit, sometimes described as a sweet-meal biscuit, is a semi- sweet biscuit (usually known in American English as a " cookie") that originated in the United Kingdom and is popular worldwide. The digestive was first developed in 1839 by two Scottish doctors to aid digestion. The term "digestive" is derived from the belief that they had antacid properties due to the use of sodium bicarbonate when they were first developed. While it is true that bicarbonate of soda is present in the biscuits in large amounts, much of it is decomposed into sodium carbonate (NaCO) during cooking, which has only negligible effects on acid production. Historically, some producers used diastatic malt extract to "digest" some of the starch that existed in flour prior to baking.
Usage examples of "digestive biscuit".
They could make a cup of tea and, if they were feeling particularly rakish, have a chocolate digestive biscuit.
His gormless face was turned towards Lobelia, who was caught by the camera as she handed Eric a digestive biscuit and a steaming mug.
While she waited for the kettle to boil she nibbled a digestive biscuit thoughtfully.