Term
Out
When the ball lands outside the court. A ball that touches the line is not out.
When the ball lands outside the baseline or the sideline (8.C). If the ball touches the line even a little, it is in (8.B, 8.E, A ball that touches the line is in).
Even a ball that looks like it is going out is not out if someone hits it, or it hits someone's body, before it lands (10.C, Even outside the court, getting hit by a ball in the air is a fault). If it hits a player's body, it is that player's fault (10.C.3).
How to read the sources: numbers like (8.F.4) are rulebook sections, and links are outside sources. An "Author's note" is the author's personal experience, not an official rule.