I have seen the terms "lines" and "veils" used on this SE (including on the chat) a number of times, especially in terms of social contracts between players.

• What do these terms mean?
• What is their origin?
• How are they frequently used (best practices)?
• What are some suggestions for introducing these terms to your group? (Without, for example, doing something that might otherwise cross a line, or be beyond a veil?)