Definition of "massless" [massless]

• Having a mass of zero. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "2. Light is virtually massless: Light is typically described as massless, or weightless, especially when it is functioning like a wave."
• "Furthermore, we can start from the idea of massless spin-two gravitons coupled to energy and momentum, and show that the nonlinear completion of such a theory leads to Einstein’s equation."
• "The discussions remind me of the massless beams, frictionless planes, and inviscid/incompressible flows that dot the physics and engineering pedagogy: very useful to a point, and wildly inaccurate past that point."