Constant time, often denoted as O(1) in computer science, refers to an algorithm's performance that remains unchanged regardless of the size of the input data. This means that no matter how many elements are processed, the time taken to complete the operation is consistent and predictable. For example, accessing a specific element in an array is a constant time operation. Whether the array has 10 elements or 10 million, retrieving an element by its index takes the same amount of time, making it efficient for certain tasks.