The term "nonic" can refer to a few different things, so here are a few definitions:
Nonic glass: A nonic glass is a type of beer glass that has a slight bulge near the top of the glass, just below the rim. This bulge is designed to make the glass easier to hold, and to prevent it from slipping out of the hand. Nonic glasses are commonly used in British pubs.
Nonic pint: In the United Kingdom, a nonic pint is a pint of beer that is served in a nonic glass (see above). The slight bulge in the glass means that the actual volume of the pint is slightly more than a standard pint (568ml vs 568.26ml).
Nonic equation: The nonic equation is a type of ninth-degree algebraic equation, meaning that it can be written in the form ax^9 + bx^8 + cx^7 + ... + jx + k = 0, where a, b, c, ..., j, and k are constants and x is the variable. Solving nonic equations is generally very difficult and can require complex mathematical techniques.
I hope one of these definitions was what you were looking for!