The function f(x) converts a single numeric character input to its corresponding English word representation. If the input is not a single numeric character, the function returns an empty output.

[CODE]:
```python
def f(x):
    numeral_to_word = {
        '0': 'zero', '1': 'one', '2': 'two', '3': 'three', '4': 'four',
        '5': 'five', '6': 'six', '7': 'seven', '8': 'eight', '9': 'nine',
    }
    return numeral_to_word.get(x, '')
```