A version of the Intel 486DX microprocessor with an extra pin, for use in the coprocessor socket of an Intel 486SX system. The 487SX provides the FPU which is missing in the 486SX.
Although the 486SX is completely disabled when you install a 487SX, the 487SX design requires that you leave the 486SX in your PC [why?], rather than use it elsewhere. Intel admits that in some systems you can unplug the 486SX and fit a 487SX in its place but they don't guarantee that it will always work.
See Intel 486.