What is the best way to describe the relationship between two companies that offer basically the same product or service?

Direct competitors.

Explanation:

Direct competitors refers to businesses that have the same offering which means that they compete for the same market. In this situation, companies will keep track of prices and also, innovations a competitor may introduce that may give a competitive advantage. What companies will try to achieve is to convince customers that their product is different and better than the one the competition has.

Because of this, two companies that offer basically the same product or service are direct competitors.

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