\<space>
For example,
Jones, et al.\ (1993) ...Such a space may also be needed after a text-producing command, such as one defined with \newcommand. As an example, consider
\newcommand{\water}{H$_2$O}
\water, \water\ everywhere, but not a drop ...
The first \water command is terminated by the comma;
note that LaTeX does not leave a space between the first \water
and the comma -- as desired -- and the trailing space is treated as an
interword space. However, the second \water command
is terminated by the space, but LaTeX would not leave an extra space after
this, running "H2Owater" into one word, except for the
trailing \ .
An alternate way to indicate the end of a command so that a trailing space is interpreted as an interword space is to use braces:
\water, \water{} everywhere, ...