Isosteres are atoms, molecules, or ions of similar size containing the same number of atoms and valence electrons.
Consider neon, the noble gas at the end of the second row of the periodic table.
To the left of it, fluorine has one fewer electron. If fluorine ionizes, it gains an electron to become F-, which is an isostere of Ne.
Similarly, O2- is an isostere of Ne and F-.
In fact, neon is part of a group of six isosteres. These are:
O2-, F-, Ne, Na+, Mg2+, Al3+
These species are isosteric: H-, He, Li+
Nitrogen, carbon monoxide and the cyanide ion are isosteric molecules; their electronic Lewis structures are identical:
The term isosteres is not frequently heard in chemistry; the term bioisostere is commonly used in drug development.