What does ||= (or-equals) mean in Ruby?

Question

What does the following code mean in Ruby?

||=

Does it have any meaning or reason for the syntax?

Answer 1

As a common misconception, a ||= b is not equivalent to a = a || b, but it behaves like a || a = b.

But here comes a tricky case. If a is not defined, a || a = 42 raises NameError, while a ||= 42 returns 42. So, they don't seem to be equivalent expressions.