It's trickier than most people think. The term "crosswalk" is American English -- it's known as a pedestrian crossing or zebra crossing in other countries. (another oft confused word is "pavement", which means sidewalk in British English, but refers to the hard surface of the road in American English)
The markings also cause confusion. To the non-American mind, does "crosswalk" refer to the crossing with the two parallel lines or the one with the stripes? Whereas with "zebra crossing", there's no ambiguity.
You don't really have to know which squares are actual crosswalks or what a crosswalk really is, reCAPTCHA doesn't care and it thinks that any street with white striped lines is a crosswalk even if the lines are for indicating something else (which is also annoying because it'll ask you to verify you are human again if you don't check those non-crosswalks). So do as I do and select any street that has white striped lines (same goes for its more rare bicycle/motorbike, small truck/car or truck/bus mix-ups).
Yes, crosswalks are a marked place on a road where pedestrians are supposed to cross. They often have dedicated signaling to let you know when you have the right-of-way to cross. The signaling usually looks like this [1], though typically the other fingers on the hand aren't blacked out, so it's a full hand, rather than one showing a middle finger (I couldn't quickly find a better example).
The question of what a crosswalk is can be surprisingly contentious despite attempts at federal standards! In my city there is a situation where the police department and the transportation department currently define "crosswalk" differently, leading to some rather bizarre situations with police reports and attribution of fault (e.g. pedestrian crossing in something the transportation department considers to be a crosswalk but the police department does not, such as a marked bicycle crossing, because their definition is stricter).
Off topic: imgur showed me the image, then gave me a cookie popup and when I clicked decline, the image was replaced by the text “could not find image”
The GP poster knows what a pedestrian crossing is, what it's for, and how it looks in most countries that can afford to build them; they were just pointing out that the word “crosswalk” is only found in American English.
I assumed it was an enclosed bridge between two buildings, a skybridge or skywalk, but apparently they are black and white striped crossings that I know as "zebra crossings".
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