The Sociologists define a caste as follows:
A caste is a hereditary, endogamous, usually localized group, having a traditional association with an occupation and a particular position in the local hierarchy of castes. The relations between castes are governed, among other things, by the concepts of pollution and purity, and generally, maximum commensality occurs within the caste.
On spatial level, a caste is usually segmented into several sub-castes and each sub-caste is endogamous. This segmentation is probably the result of a long historical process in which groups continually fissioned off. As a result of this long process of development there has come into existence several cognate groups usually found scattered over a limited geographical region, each of which retains a sense of identity as well as its linkage with other similar groups. Hence, a perception that a caste has its limited social boundaries is limited view meant for a particular time and place only. It, under historic impact, keeps changing the social boundaries and even the spatial spread through the process of historic fission.
Srinivas M. N., Caste in Modern India and other essays, 1962, Media Promoters and Publishers Pvt. Ltd. Bombay, pp 2,3. Chapter 1, Introduction.