Two meanings
The phrase has a narrow meaning and a loose one. The narrow one is the EPAL Euro pallet, which EPAL lists as 800 × 1200 × 144 mm, about 25 kg, with 11 boards, 9 blocks and 78 nails. The loose one is any pallet with the same 800 × 1200 mm footprint, branded or not.
Why the difference matters
Only the branded, licensed pallet has a published specification, a quality classification and an exchange pool behind it. A generic 1200 × 800 pallet may be well built, but a receiver who asked for EPAL will check the blocks for the EPAL mark, the control staple or the repair marking nail. When you buy or sell, write the EPAL type number in the order instead of "euro pallet".
In inches
800 × 1200 mm is 31.5 × 47.2 in. Next to a 48 × 40 in pallet it is 8.5 in narrower and 0.8 in shorter, which is why euro pallets sit awkwardly in North American racking.