Which is not a natural number?

Can you clarify for me which number in the group being discussed is not considered a natural number? I'm assuming we're looking at a set of numbers and need to identify the one that doesn't fit the definition of a natural number, which typically refers to the counting numbers starting from 1 and extending indefinitely. Are we dealing with fractions, decimals, negative numbers, or zero? I'd appreciate a clear identification of the outlier number that doesn't belong in the category of natural numbers.