olemus
probleem, millele pole lahendust
Wiktionary:
a problem that cannot be solved

näiteid
lahendumatu on võrrand, millel puudub nõutavaid tingimusi rahuldav lahend
= an equation that does not have a solution that satisfies the required conditions
võrrand x² + 1 = 0 on reaalarvude hulgas lahendumatu
x=y/0 where y can be any value, would be an unsolvable equation, since nothing is divisible by zero