The mass market is a general business term describing the largest group of consumers for a specified industry product. It is the opposite extreme of the term niche market

Niche market

A niche market is the subset of the market on which a specific product is focusing; therefore the market niche defines the specific product features aimed at satisfying specific market needs, as well as the price range, production quality and the demographics that is intended to impact...

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General

The mass market is the group of consumers who occupy the overwhelming mass of a bell curve for common household products, i.e. they could be tagged as being "average". Yet, this group consists of such a wide variety of people, their desires towards a certain product may be totally different from each other. Often competition to supply the large market is fierce, but relatively easy to enter because of the mass amount of consumer park available.

Although similar sounding, the mass market is the most unwieldy to harness by mass marketing

Mass marketing

Mass marketing is a market coverage strategy in which a firm decides to ignore market segment differences and go after the whole market with one offer. It is type of marketing of a product to a wide audience. The idea is to broadcast a message that will reach the largest number of people possible...

because of the sheer number of consumers and the myriad of needs. It is difficult to target each and every customer in the mass market, therefore, marketeers must divide the market into smaller groups with similar wants.