Auto Fill mode (@pxref{Filling}) if that's what you want.
@vindex truncate-lines
+@findex toggle-truncate-lines
@cindex truncation
As an alternative to continuation, Emacs can display long lines by
@dfn{truncation}. This means that all the characters that do not fit in
time, the default value is in effect. The default is initially
@code{nil}. @xref{Locals}.
+ The command @kbd{M-x toggle-truncate-lines} toggles the display
+between continuation and truncation.
+
@xref{Display Vars}, for additional variables that affect how text is
displayed.