Swollen eyes are not a good thing obviously and can indicate serious illness that you could spread to your flock, so isolation of this bird is crucial (if you have not done so already, do so immediately and use good bio-security for the rest of your flock until you know what you are dealing with).
Is it warm, soft and squishy? or firm?
If it is firm, it could be a tumor possibly indicating Marek's.
Warm and squishy can be anything from a swollen sinus to infected eye. Are there any other symptom like sneezing, rales, knicking, nose drainage?
If it is just that one eye, with no other symptoms, and it is warm and squishy, you may try washing the eye out with a gentle cleanser and spreading antibiotic eye ointment on it (which you can get at the feed store). Warm tea bag compresses can help alleviate the swelling and encourage drainage. If it truly feels like an abscess, you might consider aspirating it with a small needle and syringe...I personally would prefer not to mess with something like that above the eye.
However, look and be on careful watch for other symptoms as well. Swelling in the head and neck can indicate a number of viral and bacterial diseases, some of them quite nasty, and some of them requiring reporting to the county Ag authorities (Avian Influenza for example).
This honestly looks like a good job for a vet if you are willing and able due to its location and the animal is valuable to you, although I know a lot of us prefer not to take chickens to the vet.
Here is a good article on Chicken Eye Care Problems:
And a general disease resource
Keep us posted as to how the bird progresses and what your plan of care ended up being.