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based on an earlier version of the Wolfram Language.
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Styles and Fonts in Output

Style[expr,options]print with the specified style options
Style[expr,"style"]print with the specified cell style

Specifying output styles.

The second x2 is here shown in boldface.
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This shows the word text in font sizes from 10 to 20 points.
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This shows the text in the Helvetica font.
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Style allows an abbreviated form of some options. For options such as FontSize, FontWeight, FontSlant and FontColor, you can include merely the setting of the option.
Options are specified here in a short form.
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option
typical setting(s)
FontSize12size of characters in printer's points
FontWeight"Plain" or "Bold"weight of characters
FontSlant"Plain" or "Italic"slant of characters
FontFamily"Courier", "Times", "Helvetica"font family
FontColorGrayLevel[0]color of characters
BackgroundGrayLevel[1]background color for characters

A few options that can be used in Style.

If you use the notebook front end for Mathematica, then each piece of output that is generated will by default be in the style of the cell in which the output appears. By using Style[expr, "style"] however, you can tell Mathematica to output a particular expression in a different style.
Here is an expression output in the style normally used for section headings.
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"Cells as Mathematica Expressions" describes in more detail how cell styles work. By using Style[expr, "style", options] you can generate output that is in a particular style, but with certain options modified.