Originally, the term has really been just opposed to batch processing, in connection with systems like Whirlwind, SAGE, the DEC PDP-1 (as the first commercial real-time system) and is tightly connected to the idea of interactive computing. (Another early real-time system outside of the MIT tech path was MIDSAC, 1953.)
Compare Digital founder Ken Olsen's use of the term in his oral history, "The original computing was based on the way people had done computations before. You'd collect all the data, bring it together, process it and send the answers back. The idea of processing it, real time, took a long time to develop. In the world of commercial processing, it's just in the last few years that batch processing has started to disappear. The replacement for it is now called transactional processing, where if you make a transaction with a bank, it is instantly taken care of." (https://www.computerhistory.org/collections/catalog/10263035...)
There are many applications to this, each coming with their own set of implications. E.g., if you want a smoothly running interactive program on an early PDP computer, you want to have program paths of about equal runtime duration. Which may mean in turn that you would want opt out of no-operation paths as late as possible, by this stabilizing run-time, rather than as early as possible. (E.g., we perform calculations anyway, but apply the result only under a certain condition.) Or, if it is about complex processes, where it may mean that we will fulfil a contract in a guaranteed span of time to facilitate cooperations of any kind (e.g, what Olsen calls transactional processing or matching sampling rates with digital computers as a replacement for analog lab computers). Or it may be bound to a particular domain, where stale data isn't of any use (e.g., compare Whirlwind's origin in a digital flight simulator.) Or it may be just about a system being able to respond to input at all.
Compare Digital founder Ken Olsen's use of the term in his oral history, "The original computing was based on the way people had done computations before. You'd collect all the data, bring it together, process it and send the answers back. The idea of processing it, real time, took a long time to develop. In the world of commercial processing, it's just in the last few years that batch processing has started to disappear. The replacement for it is now called transactional processing, where if you make a transaction with a bank, it is instantly taken care of." (https://www.computerhistory.org/collections/catalog/10263035...)
There are many applications to this, each coming with their own set of implications. E.g., if you want a smoothly running interactive program on an early PDP computer, you want to have program paths of about equal runtime duration. Which may mean in turn that you would want opt out of no-operation paths as late as possible, by this stabilizing run-time, rather than as early as possible. (E.g., we perform calculations anyway, but apply the result only under a certain condition.) Or, if it is about complex processes, where it may mean that we will fulfil a contract in a guaranteed span of time to facilitate cooperations of any kind (e.g, what Olsen calls transactional processing or matching sampling rates with digital computers as a replacement for analog lab computers). Or it may be bound to a particular domain, where stale data isn't of any use (e.g., compare Whirlwind's origin in a digital flight simulator.) Or it may be just about a system being able to respond to input at all.