Extension:Lingo provides Mediawiki with a glossary feature. Basically if a word is in the glossary mousing over it should pop up its definition.
Prerequisites include php’s DOM module, which is normally disabled on Red Hat Enterprise Linux (RHEL) and its clones. This can be added by installing the php-xml package appropriate to the platform RHEL version.
Once installed and enabled the extension allows for creating a global [[Terminology]] page in which terms can be mapped to their definitions.
See the doc cited above for more details.