Have you seen a :Fediverse: rainbow pentagon thing on here and wondered what it means?

It's the logo of the Fediverse, the overall network which contains Mastodon, PeerTube, Pixelfed, BookWyrm, OwnCast and many other platforms that talk to each other using a common standard.

You can download a high quality version of the logo at:

➡️ commons.wikimedia.org/wiki/Fil

Technically there can be no official logo on a decentralised network, but this is the most commonly-used logo.

The Fediverse logo. It's a pentagon with large dots on each corner and lines joining all the dots to all the other dots. Each line is a different colour, giving a rainbow effect to the overall logo.
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If you have a fediverse account, you can quote this note from your own instance. Search https://social.growyourown.services/users/FediTips/statuses/114508185503289779 on your instance and quote it. (Note that quoting is not supported in Mastodon.)