green light 
noun
1A green traffic light giving permission to proceed.After all of these years of driving, after all of the red lights and green lights that I have perceived and responded to correctly, why did I do that?...
- He points out that the proposal to give left-turn cars advance green lights will slow down streetcars and probably negate any gains made otherwise.
- With the Scottish capital's new traffic regulations, a green light is no guarantee of getting anywhere in a hurry.
1.1Permission to go ahead with a project: the council has given the green light for a housing development...
- The Ministry of Mines and Energy has given the green light for the project to go ahead.
- However, the Department of Agriculture has given the green light for the event.
- Once the project is given the green light it will take one to two years to complete.
Synonyms
authorization, permission, approval, assent, consent, sanction;
leave, clearance, warranty, agreement, imprimatur, one's blessing, the seal/stamp of approval, the rubber stamp;
authority, licence, dispensation, empowerment, freedom, liberty
informal the OK, the go-ahead, the thumbs up, the say-so, the nod
rare permit, nihil obstat
authorize, give permission for, permit, sanction, allow, agree to, approve, give one's consent/assent to, consent to, assent to, accede to, countenance;
license, legalize, make legal, legitimize, legitimatize;
ratify, endorse, validate, accredit, warrant
informal give the go-ahead for, give the OK to, OK, give the thumbs up to
North American informal green-light
North American rare approbate
verb
(green-light) [with object] chiefly North American Give permission to go ahead with (a project): they refused to green-light his script until he did a major rewrite...
- RTE green-lighted the project, including a week on location in Portugal, as soon as Carney submitted six scripts.
- These days, Hollywood only likes to green-light projects they feel stand a good chance of making $20 million on opening weekend.
- It won't be easy for Chase to find a network to green-light the project.