In general, a co-occurrence is the simultaneous occurrence of two or more events.
For example, the arrival of two travellers at the airport could be called the co-occurrence of their arrivals. But this is a bit tiré par les cheveux... Hum. How do you say that, in English? I don't know. But it is clear that this example is not adapted: you never use this expression in this case.
On the contrary, in the case of the presence of two terms in the same bibliographic reference, in order to compute the influence of one term on another, you can use their co-occurrence (see influence and co-occurrence).