What substances that are formed by the chemical combination of two or more elements in definite proportions

This are called compounds

Explanation:

Compounds are substances formed when two or more elements are combined, and by definite proportions they should always be in fixed ratios. The elements can be bonded together either through covalent or ionic bonding.

In a covalent bond the atoms in the compound are sharing their outermost electrons to achieve stability, for example, CF4, CH4, CH3COOH among others. Most of the organic compounds are made of covalent bonds.

In an Ionic bond atoms in the compound are losing and gaining each others' valence electron (transfer of electrons) to form and achieve stability. For example, NaCl, KOH, CaBr2, among others. Inorganic compounds are in their majorities, ionic compounds.

We also can have metallic bonds.