a sub-set or a specific segment of a larger market which share common characteristics unique to that sub-market. 

Example: Real Estate Market is a very broad market. Lake properties are a sub-market in real estate. Fundamental issues about the size of the lake and recreation available at the lake, factors peculiar to this market include view of lake whether restricted or panoramic; access to the lake; availability of a dock permit; and depth of water at the dock. These factors have a considerable affect on the prices paid for lake properties and distinguish them from other non-lake properties.