Describes something that causes or is capable of causing death. Used in medical, technical, or serious contexts.
The poison was in a lethal dose.
I contracted a fatal disease.
Compound of 致死 (chishi, 'lethal, causing death') + 的 (-teki, '-like, -ic'). The exact historical derivation is uncertain, but the word follows a common pattern for forming adjectival nouns from Sino-Japanese compounds.