The Big Question—What is 3D Printing? - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 9

Figure 1.5 A bundle of plastic filament, ready for melting.
Most 3D printers that print in plastic use electricity to heat up a component called
the hot end . The hot end melts the plastic, and works with another component
called an extruder that pushes the solid plastic into the hot-end. Inside the hot-end,
the solid plastic melts and exits the hot-end's nozzle onto a flat surface. This is
called extrusion . This melted plastic quickly cools and solidifies.
Figure 1.6 shows an example of both an extruder and a bit of melted plastic com-
ing out of the bottom as a thin, fine string.