Used in a web form, a radio button is a selection element. It's a small circle that the user can click on. Once clicked, the radio button has a black dot in the center. Radio buttons are commonly used where there is an either/or option. When multiple options can be selected, checkboxes are more commonly used.
A broad term that describes how well a site performs. Most commonly used to refer to search engine ranking, which is the position of the site in the SERP of a particular search engine, for a particular search phrase.
One web page can have more than one ranking, for example, it can rank #1 for the search term "widget" and #3 for "gadget".
A raster image is made up of pixels, arranged in a grid. Raster images are resolution dependant - the more pixels there are in the grid, the higher the resolution and the higher the image quality. Examples of popular raster image formats include JPG, BMP, GIF and PNG.
In the context of web design, reach refers to the percentage of the total audience that the site reaches. For example, if 1 out of every 1,000 page views on the Web was on WebDesignDictionary.com, then this site would have a reach of 0.1%.
In web design the term refers to space on a page rather than real-world properties. For example, the section of the web page that is above the fold (visible without scrolling), is considered valuable "real estate", not to be wasted on irrelevant content.
A reciprocal link or reciprocated link is a link back from web site A to web site B, in return for a link from B to A. The two web site owners agree to exchange links. In theory, both web sites gain a small advantage in search engine rankings.
A redirect occurs when a web page automatically sends users to another web page. There are several ways to redirect visitors and several reasons why you would want to redirect visitors. The redirect method most commonly used is a "301 redirect". This indicates that the page you are looking for has permanently moved to the new location. It's handy for when you want to change your company domain and want to make sure people who enter your old URL still arrive at your site.
When you do a search on a search engine, the search engine algorithm attempts to measure how relevant pages in its index are to your search phrase and it displays the most relevant pages at the top.
Improving the relevancy of a page for a particular search term or search phrase is an important part of SEO. It's important to note that the SEO's job is not to make the page more relevant (the content does that), but rather to make sure that the search engine can accurately determine its relevancy.
The robots.txt file is a file in the root directory of a web site that controls spider access to the site. It can allow or disallow specific spiders access to specific pages on the site. Spiders can ignore the robots.txt file, but the reputable ones check it first and index only the pages that the robots.txt file allows them to. Notably, all of the major search engines and the Wayback Machine honor the robots.txt file.