The Swiss Alpine Club's six-step trail difficulty scale: T1–T3 is hiking, T4–T6 is alpine hiking. It rates the hardest single spot, never the average.
The T-scale is the only reason you can judge a stranger's route off the internet at all — which is exactly why it gets misread so often. It always grades the hardest single spot: a loop with five hours of comfortable T2 and thirty metres of T4 is a T4 route. The same idea sits in the colour codes used on many mountain trails — blue for T2 with no fall risk, red for T3 with possible cable sections, black for T4 to T6, where sure-footedness is mandatory; when in doubt trust the T-grade, not the colour. The second mistake is treating the grade as weatherproof: rain, fog or an old snowfield can lift a route a whole step. So progress in small increments — after solid T3 comes the lower end of T4, not T5.