Finding the width of a letter, word, or phrase

Put the word in a box, and measure the width of the box. For example,

Note that if the quantity in the \hbox is a phrase, the actual measurement only approximates the width that the phrase will occupy in running text, since the inter-word glue can be adjusted in paragraph mode.

The same sort of thing is expressed in LaTeX by:

This sets the value of the length command \gnat to the width of “small” in bold-face text.