Single-nucleotide polymorphisms, also known as SNP's (prounced 'snip') is a type of genetic variation that occurs within a population of an organism.
When a nucleotide pair differs, within a population, at a particular site on the DNA this is known as SNP. An example of this is if at a particular nucleotide site some DNA molecules have a T-A base pair. However in the same population, other DNA molecules might have a G-C base pair at the same nucleotide site . This is a SNP.
One large fraction of the human population could have one nucleotide, while another large fraction has another. Two human genomes sampled at random can differ at approximately 2.5 x106 sites (1 per 1300 nucelotides)
Many SNP's have no effect on the body, whilst others can influence disease, such as the human leukocyte antigen, and some influence response to drugs. 
- ↑ Hartl And Jones,2009:62, Genetics : Analysis Of Genes And Genomes Seventh Edition.
- ↑ Molecular Biology of the Cell, 5th ed, Alberts et al, p:464
- ↑ https://www.britannica.com/science/single-nucleotide-polymorphism