The definition is an International Labour Organisation standard and is this:
“People not in employment who have not been seeking work within the last 4 weeks and/or are unable to start work within the next 2 weeks.”
It is determined in the UK using the Labour Force Survey, a random household survey (the biggest in the country), where people are asked those specific questions (as well as many others).
I haven’t looked into this OECD report thing, but I’m a bit bemused by the article, which on the face of it seems to be saying that first they removed the economically inactive people (eg pensioners) and then calculated a new economic activity rate, which on that basis is clear nonsense.
Any claim that the unemployment rate is largely undercalculated due to economic activity rates falls over somewhat when you look at the employment rates and the economic activity rates.
I’ll take a closer look though.