ballon d'essai
/ˌbalɒ̃ dɛˈseɪ/noun
A project or piece of policy put forward to test the feeling or attitude of a person or body of people; an experimental measure or proposal, a ‘feeler’.
Origin
Mid 19th century; earliest use found in The Times. From French ballon d'essai project or piece of policy put forward to test the feeling or attitude of a person or body of people, experimental measure or proposal, transferred use of ballon d'essai trial balloon from ballon + de + essai.