\<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, ...