The function takes a string input and, if the input is a lowercase letter, it returns the position of the letter in the alphabet represented as a word (e.g., a = one, b = two). If the input is a number, it returns "zero".

[CODE]: 
```python
def approx_function(input_str: str) -> str:
    if input_str.isalpha() and input_str.islower():
        alpha_position = ord(input_str) - ord('a') + 1
        number_words = ["zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen", "twenty", "twenty-one", "twenty-two", "twenty-three", "twenty-four", "twenty-five", "twenty-six"]
        return number_words[alpha_position]
    elif input_str.isdigit():
        return "zero"
    else:
        return "undefined"
```