∞ generated and posted on 2016.01.01 ∞
Donation of an electron from one atom to another, where the atoms, now of opposite charge and therefore mutually attractive, remain spatially associated.
Ionic bonds can mean different thing within versus not within aqueous solutions. Within water an ionic bond can be quite weak, that is, easily displaced by water molecules. Absent such dissolving, though, ionic bonds can literally be "hard" as a rock, that is, tying together crystalline salts.
A related issue is that of salt bridges which are attractions between full charges such as found as a component of tertiary structure in proteins (versus the protein secondary structure associated with hydrogen bonding).
In the continuum of chemical bonds, ionic bonds represent one end, where sharing of electrons is more or less absent, whereas nonpolar covalent bonds are found on the opposite end, with more or less equal sharing of electrons occurring. Between these two extremes are found polar covalent bonds.
For additional discussion, see [Bioport].