DBE - definition by example. So the definition is not necessarily a synonym of the answer, but an example, or sub-type.
A few classics - setter for DOG, bowler for HAT - and a lot of overlap.
So Mercury = element or god or planet; Galaxy = plane or bar; Vulcan = plane or god; Mars = bar or planet. Or god, as it happens, obv.
Ximenean rules are that the definition must be qualified with a 'say'; or 'for example' or 'perhaps'; requirement starting to slip.
Cheers -
HW7