A Market is “various groups of customers.”
Marketing Management, 15th. Edition, Philip Kotler, Kevin Lane Keller.

A market should be described in terms of customer need and in a way that covers the aggregation of all the alternative solutions that customers regard as being capable of satisfying that same need.
Adapted from Marketing A Complete Guide, Malcolm McDonald, Martin Christopher.

For example, some who makes buttons should think more broadly than simply a market for all the various types of buttons, but should instead consider all the alternatives to buttons for the fastening of things such as clothes. With a broader view, products such as Velcro and zips become seen as alternatives and competitive offerings.