Definition:Empty Set/Historical Note

Historical Note on Empty Set
The concept of the empty set was stated by in his initial conception of symbolic logic.

The use of $\O$ has relevance to the early days of the computing industry, when $\emptyset$ was frequently used to mean "zero", in order to distinguish it from $\mathrm O$ (the letter O).

In the same context, the letter O was sometimes seen rendered as $\odot$, so as to ensure its being differentiated from "zero".

The latter has fallen out of use, but it is still common for mathematicians, when writing their mathematics by hand, to strike through their zeroes out of habit.