An extension is when one of waves 1, 3 or 5 is noticeably longer than the other two. Impulses with three roughly equal waves are rare: usually one pulls ahead while the remaining two come out close to each other in length.
Which wave extends
Most often the third — so much so that it has become a practical guide for counting. Less often the fifth, rarer still the first.
Hence a useful corollary: if your count makes wave 1 the longest, re-check the scale. It happens, but it is the less typical picture, and it often means that part of a larger move has been taken for "wave 1".
How to spot one
Compare the lengths of the three motive waves. An extension is not "wave 3 is bigger than wave 1" on its own, but a clear gap: one wave several times longer, the other two comparable to each other.
Lengths are better measured in relative terms than in points. On an instrument that has travelled tenfold, legs of equal "height" early and late in the history describe entirely different moves.
Why it matters for the count
An extended wave itself divides into five subwaves, and they are often clearly visible — so visible that they get taken for waves of a higher degree. That is the most common cause of a lost count: what looks like a finished 1–5 impulse turns out to be the inside of a third wave.
A tell: if your "five-wave move" fits into a single surge with no noticeable pullbacks at the higher degree, you are probably marking up the inside of an extension.
Relation to targets
The Fibonacci proportions used to estimate completion zones rest on the lengths of neighbouring legs. An extension changes the picture: after an extended third, a typical fifth is taken from the first rather than from the third, and vice versa.
Hence a practical rule: before counting proportions, decide whether there is an extension and where.
What an extension does not mean
It does not say the move will continue or that the next wave will be short. It describes geometry that has already formed: one leg came out longer than the rest.
Nor does it guarantee that the extended one will be the third. Frequency is not a rule.