A Campaign is an Intelligence Object sharing many Relationships with other objects.
According to STIX Version 2.1, a Campaign is a grouping of adversarial behaviors that describes a set of malicious activities or attacks (sometimes called waves) that occur over a period of time against a specific set of targets. Campaigns usually have well defined objectives and are often attributed to Threat Actor objects.
Campaigns can be characterized by their objectives and the incidents they cause, people or resources they target, and the resources (infrastructure, intelligence, Malware, Tools, etc.) they use.
For example, a Campaign could be used to describe a crime syndicate's attack using a specific variant of malware and new C2 servers against the executives of ABC Bank during the winter of 2015 in order to gain secret information about an upcoming merger with another bank.