A biotype is a population in which all the individuals are genetically identical. Most cheetah populations are an example of this because the cats have little if any genetic variation across their entire living species.
A biotype is also a particular physiological or anatomical characteristic of an organism adapted to and living in a specific type of environment. Organisms with a biotype are distinctive from individuals in the same species that are adapted to a different type of environment. Skin color is an example of a biotype in humans; those who evolved in and are adapted to areas that get lots of solar radiation have dark skin, whereas people who evolved to adapt to less sunny areas have light skin.
Some of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.