What is allele and it's definition?

this is the definition: one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome

Alleles are the pairs of genes occupying a specific spot on a chromosome. For example, a pair of alleles controlling the same trait, e. g. eye color: one allele codes for blue eyes, another allele for brown eyes. Simple traits such as eye color may be caused by the interaction of only one pair of alleles. But for complex traits, such as height, they are usually caused by the interactions of groups of alleles. Some alleles are dominant over other alleles, as in the case of heterozygous pairings (where paired alleles are different, in contrast to homozygous pairings where alleles are the same). In the above example, since the alleles code for different eye colors they are heterozygous.

"Gene" is used when referring to an inherited trait that is usually accompanied by a phenotype - as in "tall genes" or "bad genes" - but the proper scientific term for this is / allele/.