"A lactam (the noun is a portmanteau of the words lactone + amide) is a cyclic amide. Prefixes indicate how 
many carbon atoms (apart from the carbonyl moiety) are present in the ring: β-lactam (2 carbon atoms outside the carbonyl, 4 ring atoms in total), γ-lactam (3 and 5 total), δ-lactam (4 and 6 total). Beta β, gamma γ and delta δ are the second, third and fourth letters in the alphabetical order of the Greek alphabet, respectively."
"Lactim is a cyclic carboximidic acid compound characterized by an endocyclic carbon-nitrogen double bond. It is formed when lactam undergoes tautomerization."