The use of tiny wires that are soldered to the bare chip (bare die) on one end and to metal leads of the chip package on the other. Before the advent of flip chips and solder ball techniques, wire bonding was the traditional interconnection method to and from the chip. See flip chip
and chip package
Wire Bond vs. Flip Chip
In the wire bond method (top), the die faces up and is attached to the package via wires. The flip chip (bottom) faces down and is typically attached via solder bumps. (Image courtesy of Amkor Technology, Inc.)
This bare microcontroller chip from Microchip is approximately 4x5mm. It is mounted in this special glass package to show the wire bonds.