What are compounds that have the same empirical formula but different structural formulas called?

Isomers

For example, glucose and fructose are two common monosaccharides that have the same empirical and molecular formulaschobut different structural formulas. That is, they are both composed of the same number of atoms of the same three elements, but the arrangement of those atoms in space differs between the two compounds, and this accounts for their chemical differences. Especially with organic compounds, how the atoms are arranged in the molecule is as important as what elements are present.