The basic definition of Jakob’s Law is:
“Users spend most of their time on other sites.”
Originally created to capture the fact that users learn how to use the Web from experiences on other sites, and therefore new Web sites should offer experiences that reflect this fact, its meaning has continued to be extended and evolve as a profound statement of Web behavior.
In a newer, broader sense, Jakob’s Law now refers to the fact that since users spend most of their time on other sites, one must leverage this fact explicitly in modern online product design using capabilities like Web widgets, open APIs, feeds, and social networking apps to create presence and user touchpoints elsewhere on the Web as the well.
In this way, Jakob’s Law appropriately captures the spirit of modern Web design by going to the user, rather than making the user come to the site, the latter being a harder and more expensive thing to do.
It should be noted that Jakob’s Law, when fully leveraged in the modern sense, can theoretically improve the network effect of an online service by up to several orders of magnitude.