Simple sugar and Saccharide

Simple sugar noun - A sugar (like sucrose or fructose) that does not hydrolyse to give other sugars; the simplest group of carbohydrates.

Saccharide and simple sugar are semantically related. In some cases you can use "Saccharide" instead a noun phrase "Simple sugar".

Saccharide noun - An essential structural component of living cells and source of energy for animals; includes simple sugars with small molecules as well as macromolecular substances; are classified according to the number of monosaccharide groups they contain.

Simple sugar and saccharide are semantically related. Sometimes you can replace phrase "Simple sugar" with "Saccharide", this terms are similar.