The Four Characteristics of P-Competitive Markets and the Elasticity of Firm Demand

Here are the basics:

Many small sellers

The more sellers there are, the more substitutes the consumer has

Homogenous product

When the product is homogenous, then the substitutes are "perfect substitutes."

Sufficient knowledge

When customers know the prices offered by other sellers, they will be better able to switch -- increasing elasticity further.

Free entry

In the long run, companies may even enter the market to provide still more substitutes