A program that accepts a stream of input at one end and then displays it to the user in a scroll-locked format out the other end -- i.e. it separates the stream of text into 'pages' (screenfuls), hence it's called a pager.

You can see a pager at work by piping a process' output into it. A good demonstration under *nix systems would be "ls -R / | less", where 'less' it is the pager program being used. Under MS-DOS systems, you can do similar by "dir \ /s | more" where 'more' is the pager program.