Thanks Shawn!
I added a killall Terminal command which quits the Terminal application after opening the dictionary. Not sure it if this is the most elegant way, but it works.
open dict://{query}
killall Terminal
Thanks @Ritashugisha, unfortunately I couldn't make it work either. Basically I want to create a script that diplays the OSX Thesaurus for the selected text. Trying to get the Dictionary to open is just the first step.