A rusk is a rectangular hard, dry biscuit or twice-baked bread.
In South Africa it is considered to be a traditional food (called beskuit in Afrikaans) and is eaten after having been dipped in coffee or (less often) tea. Historically it was baked at home but there are now several mass-market versions available, the most famous probably being Ouma Rusks. In addition to the traditional "plain" and buttermilk flavours there are now numerous other flavours available, such as condensed milk, muesli and lemon & poppyseed.
The term also refers to a slice of bread baked again until it is hard and crisp (also called a Zwieback).
The word beschuit (which like Zwieback means the same as biscuit, namely baked twice) refers to the light and round rusks savoured in the Netherlands. It is customary to serve beschuit met muisjes at the birth of a baby.