In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space.[
That is, isomers are molecules that consist of the same atoms, but differently arranged, as, e.g., glucose, fructose and saccharrose (which are sometimes all referred to as different isomers of glucose).
$^4He$ and $D_2$ are not isomers - one is a helium atom, while the other is a hydrogen molecule (although with the same number of protons and neutrons). Also, one should not confuse isomers with isotopes, which are the same chemical element with different number of neutrons.
Isomers are especially common in inorganic compounts. A particular type of isomers, known as chiral molecules, are mirror reflections of each other. Although, in principle, these possess the same chemical properties, they behave differently vis-à-vis otehr chiral molecules. Notably, biological systems have preferred chirality, and failure to appreciate this fact has resulted in a pharma scandal that entered many textbooks.