Toot: An individual status update on Mastodon. Analogous to a Tweet on Twitter.
Boost: Sharing a Toot to other users who follow you, dependent upon the visibility of the original Toot and your account. Analogous to a Retweet on Twitter.
Content Warning: An optional field while composing a Toot that allows you to hide the content of your Toot behind a button with some notice text. Often used as a warning for content such as nudity, mental health, politics, and other subjects that some users would prefer to control visibility of.Some instances moderate Content Warning (shortened CW) notices more than others. They often have terms in the instance Code of Conduct as to what types of content should be hidden.
Instance: A specific server running Mastodon. Generally denoted from one another by the domain name that accounts are registered at. An instance can be made for only one user, or can be open to hundreds or thousands of users.
Federation: The ability for Mastodon instances to interact with one another. Open by default, can be modified or closed down entirely by admins.
Fediverse: An informal name for the network of sites that federate with one another, This includes Mastodon, as well as other ActivityPub or oStatus based services like micro.blog, PixelFed, Pleroma, and others.
birdsite: An often derogatory term used to describe Twitter.