green light, the

green light, the

Permission to go ahead, as in The chief gave us the green light for starting this project. This term originated in the late 1800s for the signal used by railroads to indicate that a train could proceed. It was transferred to more general use in the first half of the 1900s.
See also: green
References in periodicals archive ?
The light also affords auto traffic, which still has the green light, the opportunity to turn right.
When hit with green light, the arm stretches back out, causing the ion channel to close.
In emitting green light, the material achieves a brightness half-life of 100,000 hours (a threshold luminance of 1,000cd/m2), 150% longer.