The Free On-line Dictionary of Computing (30 December 2018):

internal field separators

    ($IFS) A predefined environment variable
   in the Unix Bourne shell whose default value is the
   three-character string containing space, tab and line
   feed.  Any string of one or more of these characters
   separates the command and each of its arguments in a command
   line.

   $IFS also tells the shell's built-in read command where to
   split an input line when reading into multiple variables.
   E.g. setting IFS=: would be appropriate for reading a file
   with ':'-separated fields, such as /etc/passwd.

   (1999-04-07)