OMEMO is an XMPP Extension Protocol (XEP) for secure multi-client end-to-end encryption. It is an open standard based on Axolotl and PEP which can be freely used and implemented by anyone.

• https://xmpp.org/extensions/inbox/omemo.html
• https://conversations.im/xeps/multi-end.html

For further reading on an iOS implemention, see this report about ChatSecure status on OMEMO.