invert sugar means: A small amount acid is used to heat sugar syrup to convert sucrose to glucose or fructose. This reduces the size of crystals. Fondant frostings are made with inverted sugar. (in Cooking Dictionary)

What else does invert sugar mean?

Hydrolysis of sucrose results in a mixture of equal amounts of sugar and fructose. This sugar is naturally found in honey and fruits, and artificially produced for the food industry. (in Merlin Dictionary)