Once popular in black SEO the way to post links that are invisible to users but indexed on the pages of websites search engine robots. The effect of "invisibility" is achieved by attaching a link to an image measuring 1x1 pixels.

Single pixel links are used for fraudulent buildup reference profile searchable sites. Typically, such links are part of the comments of popular visitors blogs, forums and portals and, unnoticed moderators, do their “black” thing - pass part page weights, on which they are placed.

As with all other black optimization techniques, single-pixel links are not approved by search engines and can cause targeted web resources to fall under their filters.