What is co-marketing?
It's exactly what it sounds like. It's when you work with another business, or group of businesses, on a marketing strategy or campaign that promotes all of your companies. Here's HubSpot's definition:
"It's the opportunity for two brands to work together on promotional efforts with a co-branded offer. In a co-marketing partnership, both companies promote a piece of content or a product, and share results of that promotion with each other."
Via Brian Yanish - MarketingHits.com