In the fall of 1967, [I was invited] to a conference ... on pulsars. ... In my talk, I argued that we should consider the possibility that the center of a pulsar is a gravitationally completely collapsed object. I remarked that one couldn't keep saying 'gravitationally completely collapsed object' over and over. One needed a shorter descriptive phrase. 'How about black hole?' asked someone in the audience. I had been searching for the right term for months, mulling it over in bed, in the bathtub, in my car, whenever I had quiet moments. Suddenly this name seemed exactly right. When I gave a more formal Sigma Xi-Phi Beta Kappa lecture ... on December 29, 1967, I used the term, and then included it in the written version of the lecture published in the spring of 1968. (As it turned out, a pulsar is powered by 'merely' a neutron star, not a black hole.)
[Although John Wheeler is often identified as coining the term 'black hole', he in fact merely popularized the expression. In his own words, this is his explanation of the true origin: a suggestion from an unidentified person in a conference audience.]