Aphotic refers to a place or situation that is completely devoid of light, especially sunlike. For example, you might describe a cave that is so dark that you can't see anything without a flashlight as aphotic.
The term aphotic is often used by marine biologists to describe a place in the ocean where sunlight cannot penetrate — the aphotic zone. The depth at which the aphotic zone begins in the ocean depends on many factors. In clear, tropical water sunlight can penetrate deeper and so the aphotic zone starts at greater depths. Aphotic comes from the Greek prefix ἀ, meaning "without", "and phōs, meaning "light."