Autom. marking specyfic words in post content

This is going to be fun and hard to explain ??
I’m looking for a way to automat. mark specific words in a post content. Lets say I have a article about something and the content looks like this:

Lorem ipsum….. the XXX is used for……lorem ipsum.

I want the XXX to be “marked” (diffrent span style?) and WP to attach a description of it at the end of the article. Of course it’s just an example.

So all in one:

• define specific words/phases to be autom. highlighted in every post content
• store and return a text description of the word/phase at the end of a article