I

Impression -

A web visitor accessing a page or banner ad.

Inbound Link -

This is a hypertext link to a page from another site, bringing traffic to that page. Inbound links are used to calculate link popularity, and are critical for search engine optimisation.

Index -

The search engine's database which stores textual content from every web page that its spider visit's.

Invisible Web -

The term that refers to the vast amount of information on the web that is not indexed by search engines.