Reflow
Content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions, for vertical scrolling content at a width equivalent to 320 CSS pixels, or for horizontal scrolling content at a height equivalent to 256 CSS pixels, except for parts of the content which require two-dimensional layout for usage or meaning.
In plain language
When someone zooms in a lot, the page should rearrange into a single column that only needs scrolling up and down — never side to side to read a line of text.
Why it matters
People with low vision often zoom in significantly to read comfortably. If that forces them to scroll both directions just to follow one line of text, reading becomes exhausting or impossible.
In practice
Fails this criterion
A news article with a sidebar doesn't adapt when zoomed in — readers must scroll right to finish each line, then scroll back left for the next one.
Meets this criterion
The same article reflows into a single column at high zoom levels, with the sidebar moving below the content — reading only requires scrolling vertically.
Test it yourself
Zoom the page in significantly (equivalent to a 320px-wide screen, e.g. 400% zoom on a standard monitor). Do you ever need to scroll both horizontally and vertically just to read the text? If so, this criterion is failing.