Links
Link exchange
Trading links with another site: you link to them, they link to you. Google's spam policies list excessive link exchanges as link spam.
Google defines link spam as creating links primarily to manipulate rankings. Its examples include buying links, exchanging goods or services for them, using automated programs to create them, and excessive link exchanges or partner pages made only for cross-linking.
The word that matters is "excessive". Two businesses that refer customers to each other and link once are not what the policy describes. The pattern that is: links placed because they were traded rather than because they help the reader, in volume, on unrelated pages, with keyword-heavy anchor text.
We run an opt-in link exchange inside our product, so weigh this accordingly. Our guide sets out what Google's policy says and where the line is.