[tialaramex]

Almost but no. Today all the SI base units are defined in terms of a universal constant, so that if you understood how SI works you can do all the same metrication work from a distant galaxy, you don't need to be here on Earth. The metre is defined by the constant c, the "speed of light"

But because of this original definition the equator will work out to about 40000 km.

[Someone]

It also never was by definition. The meter was originally defined as (https://en.wikipedia.org/wiki/History_of_the_metre) “one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth's flattening of 1/334”

You have to add the fact that earth is almost spherical to go, from there, to “the length of the equator is about 40 million meters”.

Reading that Wikipedia page, I think it already was known when the meter was defined that the polar diameter of the Earth is smaller than its equatorial diameter.

[thaumasiotes]

Well, the speed of light was measured in traditional meters and then new meters were redefined based on the existing speed of light. So the difference from 40000km attributable to redefinition would be on the order of centimeters or smaller; any deviation could only arise from our inability to measure the speed of light precisely, and we can be quite precise.

There's some difference (~7km, if we believe wikipedia) attributable to the original measurement of Earth's circumference being off, and much much more difference (~67km, ten times as much) attributable to the fact that the earth is slightly oblate. (In other words, meters were defined by reference to the polar circumference, but the equatorial circumference is larger than the polar circumference is.)

[nonfamous]

“The Mapmakers Wife” is an interesting book that tells the story of how explorers measured arcs of latitude and longitude around the world to figure all this out. The audiobook version is good too.