An acronym which stands for Search Engine Friendly in regards to website URLs. While not a Joomla specific term, it does apply to dynamic websites such as those built on Joomla. URLs from dynamic websites are often not Search Engine Friendly and need to be altered in some way to make them "friendly" to search engines such as Google.
Dynamic websites put together web pages by pulling data from a database, and often the resulting URL, or web page address, is not humanly readable. They may have a string of numbers and characters that have nothing to do with the content on the page. This is why search engines such as Google and Yahoo have trouble with them.