Noun

1. A non-existent or empty value or set of values.
2. Zero; nothing.
3. the ASCII or Unicode character (), represented by a zero value, that indicates no character and is sometimes used as a string terminator.
4. the attribute of an entity that has no valid value.

Adjective

1. Having no legal validity, "null and void"
2. insignificant
   • 1924, Marcel Proust, Within a Budding Grove:

     In proportion as we descend the social scale our snobbishness fastens on to mere nothings which are perhaps no more null than the distinctions observed by the aristocracy, but, being more obscure, more peculiar to the individual, take us more by surprise.

3. absent or non-existent
4. of the null set
5. of or comprising a value of precisely zero
6. causing a complete loss of gene function, amorphic.