In a word
Definition 1 of 1
Pro Tip 1/3Concise Summary
Use 'in a word' when you want to express a complex idea simply and quickly. 
The hotel was, in a word, luxurious.
Pro Tip 2/3Transition to Summary
'In a word' acts as a bridge to a succinct conclusion after a lengthy explanation. 
I could go on about the science, but, in a word, it's groundbreaking.
Pro Tip 3/3Dramatic Effect
This phrase adds emphasis and can create a dramatic or impactful summary. 
The play was, in a word, mesmerizing.