Oxford Dictionary says: "the principle that the speed and capability of computers can be expected to double every two years, as a result of increases in the number of transistors a microchip can contain."
Which notably mentions performance. I have often interpreted Moore's Law to be the fact that doubling improvements in performance will continue roughly every 16-18 months, but not always directly proportionally to transistor count -- other ways to squeeze out doubling performance gains arise and often in unexpected places.
Oxford Dictionary doesn't know what Moore's law is, then.
Moore's law was an observation by Gordon Moore, co-founder of Intel, relating to the doubling of transistors per densely integrated circuit chip every 24 months. Originally, it was every 12 months, then every 24 months, etc.
Moore's second law, which isn't as well known, relates to the exponentially increasing capital cost of manufacturing ICs.
Single threaded performance and overall performance are quite different. But I'm not sure that Oxford is the most authoritative source here, as they are talking about a "computer" which is an awfully vague term. Wikipedia uses transistor count of an IC, which is much more specific:
It has always referred to the minimum of the cost-complexity curve for integrated circuits. The progress of Moore's Actual Law has never stopped.
The thing people mean when they are talking about the deceleration of single-threaded CPU performance is actually Dennard scaling. Dennard said that density and power efficiency were complementary in such a way as to keep areal power constant. That was true until it suddenly stopped being true. If Dennard scaling had continued you'd be using a 20GHz CPU right now.
