The definition has changed and is changed, the best description would be that they are "programmable hardware" based on architecture that is "scale-able" like an array. This scaling in 2 directions makes FPGA's different from classic CPLD's where prices of the silicon go up very quickly when the CPLD goes larger. This is the reason why classic CPLD's are vanishing - they are being replaced with small FPGA's that fill the gap. Altera MAX 2, V, 10, Lattice XO2, XO3 can be considered as CPLD, and they target some CPLD markets, but they are actually small FPGA's as of the architecture.

Simple logic circuits are arranged in 2 dimensional array, with programmable interconnect. Very large FPGA's are 3D in that sense that those 2D arrays are layered with additional "vertical" interconnections.