Between or among; mutually or reciprocally.
[Old French entre- or Latin inter, between, among.]
Many examples derive from Latin words already containing the prefix: interest (Latin interesse, differ, be important, from esse, be); intercept (Latin intercept-, caught between, from capere, take); internecine (Latin internecinus, from necare, to kill).
Based on this model, the prefix has became a common and active one in English, forming adjectives (intercontinental, interfaith, intermolecular, interwar), nouns (intercom, interface, Internet), and verbs (interoperate, intermarry).
In the sense of ‘between or among’, examples are intercity, interglacial, and international; in that of something mutual or reciprocal, examples are interdependent, intermingle, and interrelation. However, these senses are often not clearly distinguishable in any given case.