Reach (AdWords term) 
A measure of the total number of unique users who will see an ad over a specific period of time.

Reciprocal Link (link exchange, link partner)
Two sites which link to each other. Search engines usually don’t see these as high value links.

Redirect
Any of several methods used to change the address of a landing page such as when a site is moved to a new domain or a page is withdrawn from a site’s structure. Redirects are generally not good for search engine rankings, as they dilute PageRank. There is also the risk that the search engine spider will not follow the redirect.

Referrer 
Whoever delivered a visitor to your web page is a referrer. For example, if Google's search results had a link from Yellow Pages and YP contained a link to a page on your site and a user clicked on that link, Yellow Pages becomes the referrer.

Registrar
The company which allows you to register a domain name.

Re-inclusion
If a site has been penalized for spamming they may fix the infraction and ask for re-inclusion into the index. Depending on the severity of the infraction and the brand strength of the site they may or may not be added back into the search index.

Repeat Visitor 
A repeat visitor is a single individual or browser who accesses a website or webpage more than once over a specified period of time.

Resubmission
Submitting your web page to search engines after you've already submitted the URL’s previously or after the search engine has already included your site in its index. Search engines don't like it when you resubmit as it simply clutters their queue with duplicate requests.

Robots.txt
A file in the root directory of a website use to restrict and control the behaviour of search engine spiders.

ROI (Return On Investment)
One use of analytics software is to analyse and quantify return on investment in online advertising.

RSS
Rich Site Summary or Real Simple Syndication is a method of syndicating information to a feed reader or other software which allows people to subscribe to a channel they are interested in.