Simple Definition:

An ionic bond is a chemical bond between two atoms that results from the attraction between oppositely charged ions [ions are atoms or molecules that have gained or lost electrons].

For example: Sodium [a metal] and Chlorine [a nonmetal] can form an ionic bond to create salt [NaCl]. In this bond, sodium gives up an electron to chlorine, forming a positively charged sodium ion and a negatively charged chloride ion that are attracted to each other and form the bond.

Very Simple Definition:

An ionic bond is when two different atoms stick together because one gives an electron to the other. This makes one atom positively charged and the other negatively charged, and they stick together because opposite charges attract. For example, table salt is made of sodium and chlorine sticking together with an ionic bond.