definiendum
noun
de·fin·i·en·dum di-ˌfi-nē-ˈen-dəm 
plural definienda di-ˌfi-nē-ˈen-də
: an expression that is being defined
Word History
Etymology
borrowed from Latin dēfīniendum "something to be defined," from neuter gerundive of dēfīnīre "to mark the limits of, determine, define"
First Known Use
1871, in the meaning defined above