Accounting conservatism is covered in full in the entry Conservatism, which explains the principle, its history and the debate around it. This page gives a short definition and explains the two names.
Accounting conservatism is the practice of choosing the more cautious option when a figure is uncertain. A company records a probable loss as soon as it can estimate it, but waits until a gain is actually realised before recognising it. The rule shows up in measures such as valuing inventory at the lower of cost or net realizable value and in the different treatment of a loss contingency and a gain contingency. The aim is to stop managers overstating assets and profits.
Accountants have long called this simply "conservatism", and older textbooks list it among the core accounting principles. "Accounting conservatism" is the longer name used in academic research and by readers outside the profession, so both terms describe the same thing. Modern standard-setters prefer the word "prudence" and stress neutral rather than deliberately pessimistic reporting, a shift explained in the main entry.