Dictionary.com defines jargon like this:

1. jargon: -noun, the language, esp. the vocabulary, peculiar to a particular trade, profession, or group: medical jargon.

It also defines jargon like this:

2. jargon: -noun, unintelligible or meaningless talk or writing; gibberish.

Joomla!'s roots go back a few years, and it definitely has a language all its own. Seasoned veterans are fluent in Joomla! jargon, but for "newbies", that second definition is probably the most appropriate definition for what Joomla! jargon really is. For that reason, I have decided to attempt to compile a list of the most commonly used words and/or phrases in Joomla!, and give them a definition that actually means something to a non-veteran Joomla! user.

The List

Component:

A component can be thought of as an application within an application. Components are applications that are built upon the Joomla! framework. A good analogy (at least for me) is to think of Joomla! as an operating system like Windows, and components as programs that run on the operating system. Going with that analogy, I like to think of the Joomla! content component (com_content) as the word processor (like Microsoft Word).

Only one component is loaded in your Joomla! website at a time, and it handles the major tasks that occur on a given page in your site.

Module:

A module further extends your Joomla! site by performing side functions outside of the component. Modules can be used to perform just about any task you can think of. They are typically used to display menus, lists, banners, and other little tidbits of content. I like to think of modules as being like little widgets that perform a simple, specific, necessary task that cannot be handled efficiently by components.

Module Position:

A module position is a location within a template that is set aside for containing modules. In Joomla!, the pre-defined module positions are given descriptive names like "left", "right", "header", "footer", "top", "bottom", etc., but those positions could be at any location on the page depending on where the template puts them.

Plugin:

Plugins have a variety of uses, but they typically perform a specific function to extend the functionality of a component. In Joomla! 1.0.x, there are five different types of plugins: authentication, content, editor, button, extension, smart search, installer, quick icon, search, system, two factor authentication and user.

Template:

Many people think of a template as their website, but that is an incorrect way of seeing templates. Templates are used for providing a layout and design within which all of the pieces of your Joomla! site come together.

Here's an analogy that might help. Think of your Joomla! website as a house. It is made up of brick, stone, wood, dry wall, etc. The template is the blueprint that puts all of those pieces in the most logical locations for what your house needs to be, and it is also the decorations that make your house look nice once it is completely built.

Extension:

An extension is a component, module, plugin, or a template.

Content:

Technically, content is every piece of text and media that is distributed/displayed via your site, whether that be in the component, module, plugin, or template. In Joomla!, when someone says content, they are probably referring to the core content component and the content items that are entered into that component and displayed on your site.

Category:

A category is a main level in the Joomla! content organization hierarchy. Categories can contain (but don't have to) sub-categories withing themselves, as well as content items. 

SEF URL:

This stands for "Search Engine Friendly URL". Basically, this is just a way to make your URL's look nice. The default SEF URL functionality in Joomla! takes URL's that look like this "index.php?option=com_content&task=view&id=40&ampitemid=29" and makes them look like this "content/view/40/29/". There are also 3rd-party extensions that can be used to make the URL more descriptive of the content it is pointing to.

Core:

Core refers to that basic Joomla! installation without any 3rd-party extensions installed. It is all of the code for the Joomla! framework along with a set of core extensions that come with the Joomla! installation.

3rd-Party:

3rd-party refers to any extension that is not part of the core.

Access Level:

Access level is the level of access that visitors to your site have to content in your site. Currently, the access levels available in Joomla! range from a non-registered guest all the way to super administrator.

Did I forget something?

It's late, and I'm sure that I have forgotten something. If there is a word or phrase that you see all the time in Joomla!, but you don't know what it is, please let me know, and I'll add it to the list.