Ax

The market maker who is most central to the price action of a specific security. The ax can be identified by spending several days studying level II quotes and noting which market maker seems to have the greatest effect on the security's price.

Identifying the ax is a common strategy among day traders, and trading in the same direction as this market maker can drastically increase a trader's odds of making successful trades. Any given stock can have several market makers, and the ax for any given security can change every day. It may take a trader several days to determine which market maker is primarily controlling the action of a certain stock.