Market Segment
  
See: Market Segment - Situational Analysis.
Part of a market. Take a market defined as "beverages": anything you could buy that you can drink (safely). Market segments might include "soft drinks" and "alcoholic beverages" and "bottled water" and "aged fruit-flavored kombucha."
Think of it in the same way that animals are classified. You can have big groups, like mammals or fish. These count as your markets. Then you have smaller groups, like cats or sharks. They count as your market segments. Then you have your individual species, things like tigers or hammerhead sharks. These count as your individual companies or products.
A market segment is a way to subdivide a market in order to examine it more closely.