A co-investor is an investor who participates alongside the lead investor in a financing round, typically on the lead's terms. Sometimes called a "follower" or "participating investor," they accept the same price, same documents, and same major rights, contributing capital to fill out the round but not setting the price or negotiating substantive deal terms. They are the structural counterpart to the lead investor in lead-and-follower deal structures. The co-investor role is what most investors play in most rounds; only one investor per round is the lead.
The structural distinctions: the lead investor anchors the round with the largest single commitment, sets the price (the valuation, the deal terms), negotiates the term sheet, ...