TextStyle is an option for graphics functions and for Text which specifies the default style and font options with which text should be rendered.
The following forms of settings can be used:
The options that can be given are as in StyleForm.
"style" settings can only be used when a notebook front end is present.
The default setting is TextStyle :> $TextStyle.
The style specified by TextStyle in a graphics object is used by default for all text, including labels and tick marks.
See The Mathematica Book: Section 2.9.15.
See also: StyleForm, $TextStyle, PlotStyle, FormatType.