I reported this problem 5 years ago in MDL-23535 point #1:
1.- There is no reason why the text of the definition should be displayed centered. It should normally be displayed left-aligned (or, I suppose, right-aligned for RTL languages).
For what it's worth you might like to add your vote.
Hi Murray,
I had the same issue, and found shorter definitions look better centred anyway and longer definitions needed some alignment still. Targeting the CSS for ALL glossary terms doesn't always work with every theme I've found but for each Glossary 'item' if you wrap your definition like this...
<div style="text-align: left";> Your Definition Here </div>
You can do it for the terms that really need it anyway.
This CSS will force it to align on pop-up.
Hi Murray - I've done this in the Clean theme by adding custom CSS. See this post: http://turn-editing-on.blogspot.com/2015/06/left-align-glossary-tips.html