# Glossary

It's beneficial to become familiar with key concepts that you will encounter in the documentation.

This section lists these common terms for easy access & learning.

In an attempt to add a little clarity, we will use the example of a web-based email client (like Gmail).

# Action

An action is a single, isolated piece of functionality provided by your system.

# Actions by example

ShowInbox
SendEmail
SaveDraft
ApplyLabelToConversation

Learn more about actions.

# Feature

A feature groups multiple, related actions together.

# Features by example

ComposingEmails – which might have actions like:
- WriteNewMessage
- AttachFileToNewMessage
- SendMessage
- SendMessageLater
- DeleteNewMessage
ManageConversations – which might have actions like:
- ListConversations
- MarkConversationsAsRead
- ArchiveConversations
- ReportConversationAsSpam
- DeleteConversation

Learn more about features.

# Availability

Availability determines if an action can be performed. It's built into the heart of Lantern and provides one of the biggest benefits.

You can configure when an action is available, and when it is not.

By default, an action is available unless you configure it otherwise.

Learn more about availability.

# Constraint

System-level constraints can be placed on a feature or action.

Constraints are checked along with availability and it's a great way of switching off groups of actions or features depending on what the current system setup will support.

Learn more about constraints.

# Availability and constraints by example

Let's say your Gmail-like software has an action allowing the user to DownloadAllAttachments.

You might configure your DownloadAllAttachments action like this:

availability – only allow if the current message has 2 or more attachments (this is not a system constraint, since it changes based on the current message)
constraint – only allow if the ZipArchive class exists