Terminologies of “Process calculus” and “Process algebra” [duplicate]

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• What are the justifications and historical reasons regarding the choice between the words 'calculus' and 'algebra'? 1 answer

In the literature, the terms of "process calculus" and "process algebra" are often interchangeable. Meanwhile, it confused me.

My questions are:

1. Are there formal, standard, and widely-accepted definitions of "calculus" and "algebra"?
2. What are the significant distinctions (maybe informally) between "calculus" and "algebra"?
3. What are the features that make a theory kind of "calculus" or "algebra"?