Day Master
The heavenly stem of your day pillar: the single character that stands for you.
In one sentence
The heavenly stem of your day pillar: the single character that stands for you.
How it is used
Every other character is read in relation to it: which element feeds it, which it controls, which controls it. That relation is what produces the Ten Gods, so the Day Master is the reference point rather than a personality label on its own. Two people with the same Day Master can read very differently once the season and the rest of the chart are in.