Defining it, however, is a bit harder. Ask anyone, even professional researchers who are supposed to know about such things (and therefore be very intelligent themselves) and you’ll get a very disparate set of views.
The question of intelligence is coming to the fore because we want to make our machines intelligent (presumably that would give us license to relax and be a bit dumber). It’s an incredibly hard problem. To solve it we will need to solve the intelligence problem. There has been some progress, but we’ve still got a long road ahead.
It’s hard to think about intelligence without taking into account IQ tests. They have a relatively long history, some say going back to imperial China, but they certainly at least to the 19th century. Today, we all take them at an early age to determine our aptitude.