The words 'terminus' and 'terminal' are used rather loosely by IR.

In general, 'Terminal' is the word used for any building/station from which vehicles (buses, trains) etc originate. Usually, the name 'Terminal' is associated with a grand building. So, technically, any station with lot of originating trains and an associated large building can be called a 'Terminal'.

Terminus...
more...

generally refers to a station that is at the end of the line. It can also refer to a grand building (like Victoria Terminus etc).

Both 'Terminus' and 'Central' are vestiges of the British era, and were often used to indicate some kind of grandeur or importance.
When railways were first being constructed in the country, Bombay, Calcutta, Madras and Lahore were by far the largest cities with no other city being even comparable to these places (except for Delhi). So, the stations in these cities were among the most important and largest ones in the country. So, each railway company followed different conventions in naming the stations and trains.

For...
more...

example, the GIPR (which constructed the first railway in Bombay and essentially all of CR today) and the Madras railway company (all of SR BG) always gave importance to Bombay and Madras respectively. All the trains going towards these cities were designated 'Up' and going away were designated 'Down'. When they constructed the large stations in both cities, they named them in such a way to indicate their importance.

Usually 'Central' referred to a grand building which acted as the main station for a city. This station would essentially be the 'pride' of the railway (thus Madras Central). The word 'Terminus' is also used to indicate some kind of importance affixed to the building. Since the Victoria Terminus building was kind of a showcase for the Raj, it was named 'Terminus' to give some kind of hype to the name.

The Travancore kingdom followed the Madras railway's trend and named the main station in Trivandrum as 'Trivandrum Central'. Similarly, Cawnpore was a major junction for several railway companies and a critical military centre, so the railway companies followed the same trend and named the station as Kanpur Central.

Mangalore Central was named so because some railway officials felt that it made the station sound very important. No other reason. Just someone who was fascinated with the terminology.