A pure play is an investment, usually an equity, that gives investors exposure only to a particular industry or product market.
A typical reason for buying a pure play is that an investor believes that a particular industry will grow. For example, an investor who believes that mobile telecoms will grow at the expense of fixed line telecoms may look for a pure play mobile operator, rather than a telecoms company that owns both fixed and mobile networks.
Although buying pure plays may seems to reduce diversification, investors should look for diversification in their portfolios as a whole, not at the company level. If anything, pure plays help manage risk by giving investors more control over what they are exposed to.