An html link that has gone bad. The destination page no longer exists. Many search engines routinely check for "dead links" by spidering the page again. Dead links used to be a serious problem on search engines (mostly yahoo), but with increased link checking, dead links are becoming more rare.
 

Defaults

Advertising term used to describe when an advertiser doesn't have enough advertisments to fill the websites inventory. Usually defaults are filled with PSA ads, or blanks.
 

Direct Hit

A click through counting system that counts users clicks on various search engine results. The count of clicks is then used to determine web site rankings in results pages. This system is can be manipulated quite easily.
 

Directory

A directory is a web site that focuses on listing web sites by individual topics. A quasi table of contents. A search engine lists pages, where a Directory (such as Yahoo or The Open Directory Project lists websites).
 

DNS Lookup

Or sometimes referred to as Reverse DNS Lookup. Most often used by webmasters while looking at server log files. It converts a unique IP address of a site visitor to its domain name..
 

Dogpile

A meta search engine.
 

Domain

There are Top Level Domains (such as .com, .net, or .org), and then there are midlevel domains such as Ford (ford.com ford.net or ford.org). Domain is a generic term to describe any of these levels and is most often used to refer to the mid level domain (ford.com). In reference to search engine technology, domain names can play an important part in determining a sites rankings on the search engines..
 

Domain Name Registration

The act of registering a domain name with an approved registrar. The process is overseen by ICANN. 
 

Doorway Page

A page designed as an entrance to a website. Many doorway pages a specifically created to rank high on a particular search engine. Sometimes referred to as a Gateway Page or a Bridge Page.
 

Download

The process of retrieving information from any computer is called Downloading. When one computer sends information to another, it is called Uploading.
 

Dynamic Content

A page that is generated just as the user views it. The content delivered to the user is often updated on-the-spot out of a database or based upon the users browser. It used to be easy to spot one of these pages, but with most systems now allowing dynamic content from any page at any time, you just never know. Search engines no longer penalize for dynamic content as long as the URL does not include submitted data (a ? question mark in the url).
 

Dynamic IP Address

An IP address that changes each time you connect to the internet..