Invisible Text/Hidden Text
Black hat types who wanted to put out readable content, and get the benefits of keyword stuffing have long turned to hidden text.
Hidden text is, as it sounds, groups of keywords that hidden on a page by making them the same color as the page itself. When search bots spider the page they read the hidden keywords as if they were actual content.
Of course today’s search bots are intelligent enough to suss out quality content so, whether it’s visible or not, keyword stuffing just isn’t a good technique.