"In-place" is a seemingly informal adjective used to describe algorithms. Does it have a precise definition?
To further clarify the discussion, what models of computation can we say are in-place? We can probably say whether a specific C program running on a specific machine is in-place. Can we say whether a C program is in-place, independent of its implementation? Can we say whether a Turing machine is in-place? Can we identify in-place algorithms with known complexity classes?