The function f(x) takes a string input and returns the English word for the number of unique characters in the input string.

[CODE]:
```python
def f_approx(x):
    unique_chars = set(x)
    num_unique_chars = len(unique_chars)
    num_to_word = {
        0: "zero", 1: "one", 2: "two", 3: "three", 4: "four", 5: "five", 
        6: "six", 7: "seven", 8: "eight", 9: "nine", 10: "ten"
    }
    return num_to_word[num_unique_chars]
```