라

short "because it's" or "call it"

A clipped form that hangs off a noun and does one of two jobs. As a reason it means "since it is": 주말이라 사람이 많네요 is "it's the weekend, so it's crowded", 공사장이라 is "because it's a construction site". As a shortening of -라고 before 하다 it means "call it": 케이크라 했으면 is "if you called that cake", 능력이라 해야 되나? is "should I call it an ability?". Its most common appearance by far is inside 아니라, "not X but Y", which sets up a correction. It is a compressed, conversational shape rather than a separate word you can spot on its own.

Register: conversational, compressed