'Terminal' is the general term for any building/station which has a lot of originating/terminating traffic. In that aspect, RJPB is called a 'terminal' as it was intended to de-congest PNBE by shifting some originating traffic here.
'Central' station was a term commonly used in the early British period when major cities like Bombay, Madras etc had many suburban stations. In those cases, the main station serving the city would be called 'Central' station. Note that this was just a practice followed by some railway companies of that time. Calcutta and Delhi never had any 'Central' stations. It was just the choice of some railway companies to have a grand, main station in a major city called...
more... a 'Central' station.
Terminus typically refers to a dead-end station (no tracks beyond) or the last station under the jurisdiction of a particular railway company/zone.
Note that any station which has a considerable amount of originating traffic can be called a 'terminal'. This is not a strict definition and is not always followed.
Howrah is not a junction. It is only listed as a junction in NTES. RBS, old railway schedules - none of them ever refer to Howrah as a junction. If I am not wrong, even the station boards do not have the 'Junction' written on them.
But yes, Howrah can be called a 'terminus'. As I mentioned earlier though, this is not a rule. The East Indian railway (predecessor of ER) did not bother to name the Howrah station as 'Howrah Terminus' or 'Howrah Central', while the GIPR (predecessor of CR) and Madras Railway companies wanted to have a grand building as the main station and while the GIPR constructed the 'Victoria Terminus' in Bombay, the Madras railway constructed 'Madras Central' station in Madras.
There is no clear distinction between these terms and they can be freely interchanged in most occasions.