escape hatch (n.)

• alternative
• bolt-hole
• catch
• choice
• clause
• condition
• given
• grounds
• joker
• kicker
• lifeboat
• lifeline
• loophole
• obligation
• parameter
• prerequisite
• pretext
• provision
• provisions
• proviso
• requisite
• slide
• specification
• stipulation
• string
• terms
• ultimatum
• underground