Is there a definition of light/medium/heavy aircraft?
Section 4.9.1.1 of ICAO PANS-ATM-Doc 4444 Air Traffic Management defines LIGHT, MEDIUM &amp; HEAVY aircraft for the purpose of wake separation:

4.9.1 Wake turbulence categories of aircraft
4.9.1.1 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certified take-off mass as follows:
a) HEAVY (H) — all aircraft types of 136,000kg or more;
b) MEDIUM (M) — all aircraft types less than 136,000kg but more than 7,000kg; and
c) LIGHT (L) — aircraft types of 7,000kg or less.
4.9.1.2 Helicopters should be kept well clear of light aircraft when hovering or while air taxiing.
Note 1.— Helicopters produce vortices when in flight and there is some evidence that, per kilogram of gross mass, their vortices are more intense than those of fixed-wing aircraft.
Note 2.— The provisions governing wake turbulence minima are set forth in Chapter 5, Section 5.8, and Chapter 8, Section 8.7.3.