Interchange+, also written Interchange Plus or IC+, is the short name for interchange-plus pricing: a way of charging merchants for card payments in which the acquirer passes through the actual interchange on each transaction and adds its own fixed markup. The full explanation, with a formula and a worked example, is on the Interchange-Plus Pricing page.
The "plus" is the acquirer's markup, the only part of the price it sets. A more detailed version that also lists the card network's scheme fees on a separate line is called interchange++ pricing. Both are alternatives to flat-rate pricing, where every card costs the merchant the same.