It's not even considered mathematically (3,6)
I believe the answer is:
odd number
'mathematically' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'it's not even considered' is the wordplay.
'it's not even' becomes 'odd' ('odd' is the opposite to 'even').
'considered' becomes 'number' (I can't justify this - if you can you should believe this answer much more).
'odd'+'number'='ODD NUMBER'
Can you help me to learn more?
(Another definition for odd number that I've seen is " 2017, say".)