Alone in a given category
Without equal or rival
To the exclusion of anyone or anything else
Nothing more than
I only wish there was a real horse here for me to race with.
Only Nick noticed her standing in the door.
We can only do so much.
In despair she had dropped the subject for the time, only to renew it at the first opportunity.
If he had been the only child in the family, things might have been different.