Polymorphism:  

(chemistry) the existence of different kinds of crystal crystal structures at different temperatures or pressures for a single compound
(biology) the existence of two or more forms of individuals within the same animal species (independent of sex differences)
(gentics) difference in DNA sequence among individuals. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis. Compare mutation. Differences in DNA sequences that occur naturally in a population. Single nucleotide substitutions, insertions and deletions of nucleotides and repetitive sequences (microsatellites) are all examples of polymorphism. The position at which such a sequence difference is found is a polymorphic site. A single nucleotide substitution is called a single nucleotide polymorphism or a SNP.