Which of two.
"We need to decide whether we should go to the movies or stay home tonight."
Which of two.
"I cannot choose whether to take the bus or walk because I have forgotten my shoes."
Introducing a direct interrogative question (often with correlative or) which indicates doubt between alternatives.
"Whether he will come or not remains uncertain."
Whether comes from Old English hweþer, which originally meant "which of two" and is derived from the root for "who." This word shares a common ancestor with related terms in other Germanic languages that also refer to choices or specific groups.