xterm 1) is the terminal emulator 2) that was originally developed as part of the X display server called also X(7)
xterm (pronounced eks-term) is a specific program, not a generic item. It is the standard X terminal emulator program.
Putty and Mintty uses Xterm as terminal.
For instance, when you start the Cygwin X server automatically an xterm session appear and you can make a connection to a Linux Operating system with ssh in order to show the remote client on your local system.
You can get Xterm in the browser via the xtermjs project.