Glucose syrup
A thick, clear sugar syrup made from starch, only mildly sweet, that keeps sugar from crystallising.
Glucose syrup is sold in the baking aisle of larger supermarkets, in baking and confectionery supply shops, or by mail order. It keeps caramel, turrón, ice cream and candied fruit smooth and stops sugar from turning grainy.
Substitute:
Light corn syrup behaves almost identically. Invert sugar syrup or a mild honey also work, but they are sweeter and brown faster, and honey adds its own flavour.