Good luck finding a book on classical architecture that doesn’t mislabel one or both of these features, so what’s the difference? You’ll kick yourself…once you look at the roots the answer seems obvious.
In this corner: an arcade is defined as “a series of arches supported on piers or columns”, or as “an arched or covered passageway”, or as “where I played Space Invaders in 1985”. As an aside, I recently went back to the Brunswick Lanes in Aurora where I spent countless hours of my childhood playing video games as my parents bowled and I was crushed to learn that while Galaga was still there my high score from the 4th grade had not held up as expected.
And in this corner: a colonnade, not surprisingly, is defined as “a series of regularly spaced columns supporting an entablature”.
It’s easy to see why the mix-up is so common since many examples of Roman and Romanesque architecture contain arcades which are defined or bounded by colonnades.
Bonus match: Elicit vs. Illicit
Elicit is a verb defined as “to draw out or bring forth” and illicit is defined as “not legally permitted or authorized”. So the illicit behavior of tampering with a perfectly respectable high score on a video game will elicit a weepy response.