From TeX by Topic (section 19.1.4 The number of lines of a paragraph, p 178):
Deviations from the optimal number of lines, that is, the number of lines stemming from
the optimal way of breaking a paragraph into lines, can be forced by the user by means of
the \looseness
parameter. This parameter, which is reset every time the shape parameters
are cleared (see Chapter 18), indicates by how many lines the current paragraph should be
made longer than is optimal. A negative value of \looseness
will attempt to make the
paragraph shorter by a number of lines that is the absolute value of the parameter.
TeX will still observe the values of \pretolerance
and \tolerance
(see below) when
lengthening or shortening a paragraph under influence of \looseness
. Therefore, TeX
will only lengthen or shorten a paragraph for as far as is possible without exceeding these
parameters.
An example of this, in action (and where it was possible to adjust the number of lines:

\documentclass{article}
\usepackage{geometry,lipsum}
\geometry{margin=1in}% Just for this example
\begin{document}
\lipsum[1]
\looseness=-1% Try one fewer line for the following paragraph
\lipsum[1]
\end{document}