"Reentrancy (re-enter is possible)" means that a certain process can be called concurrently at the same time from multiple processes. A reentrancy function can be correctly executed in another process while the function is being executed. For example, in an application using a real-time OS, this function is correctly executed even if dispatching to another task is triggered by an interrupt while a certain task is executing this function, and even if the function is executed in that task. When data variables or RAM are shared by multiple functions, those functions may not necessarily be reentrant.