A constant time function is one which takes the same amount of time to execute, irrespective of the size of the input that is passed into the function:

function square(num) {
    return num*num;
}

The preceding code snippet is an example of a constant time function and is denoted by O(1). Constant time algorithms are the most sought out algorithms for obvious reasons, such as them running in a constant time, irrespective of the size of the input.