There is no absolute notion of "lightweightness": by definition, this is a relative notion: on a given architecture, a specific algorithm may be "lighter" than another. Depending on the usage context, this may be "light enough" or not. Moreover, there are several metrics:
- On software platforms, we consider CPU usage and RAM usage (both L1 cache for code, depending on code size, and L1 cache for data, exercised by lookup tables).
- On hardware platforms (FPGA, ASIC...), silicon area, latency, bandwidth, enery consumption... are important values and neither is "more important" generically than any other.
An algorithm can be lighter than another for one of these metrics, and heavier for another. For instance, with typical implementations, on software platforms, AES will use less CPU than 3DES, but will need from RAM (about 4 kB for constant lookup tables; usually, this cache usage is perfectly tolerable, which is why AES is almost invariably "much faster" than 3DES).
Every researcher eager on touting the performance characteristics of his pet cipher will define his own formal line of weight, his creation being, of course, in the "light" side.