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Although L3 switch and multilayer switch are generally considered the same thing now adays (as noted by the other posters), a L3 switch does multilayer switching (MLS), but a multilayer switch might be incapable of L3 routing itself.
A good example of this, a multilayer switch, was the Catalyst 5000 series. The 5000 could do MLS in conjunction with a standalone router (while it had no on-board L3 routing capability - i.e. so the 5000 without its optional RSM wasn't a single device L3 switch). It could also do L3 routing (with its optional RSM) while not doing MLS, so then it wasn't acting like a L3 switch. Only when you configured MLS with its optional RSM did you have it operating as what we today consider a single device L3 switch.
