OBI: Observation Interval
An Observation Interval (OBI) is an uninterrupted observing interval for a single ObsId. An ObsId may be broken into more than one OBI for a variety of reasons, one of them being that the original observation did not get enough good time.
An in-depth discussion of multi-OBI observations and how to analyze them can be found in the Multi-OBI Observations why topic.
There are many ways to refer to an observation:
Proposal Number maps to N x Sequence Numbers maps to M x ObsIDs maps to Q x OBIs
where it is almost always true that N == M (i.e. there is one ObsID per Sequence Number).