Usually an organism phenotypically affected in one or more characteristics by a mutation in a particular gene or chromosome. A mutant is not the same as an organism with a developmental abnormality, the latter arising during morphogenesis. Furthermore, a mutant may prove to be lethal in one condition, but not under another.
See Character, Chromosome, Gene, Genotype and phenotype, Morphogenesis, Mutagens, Mutation (biology), Organism