The Albino Lilac
Lilac trees are supposed to be purple, or “lilac”, and indeed we have a number of such plants in our yard.

This view is looking north across much of the property toward the house – the trees in the row are Eastern Redbud, a lilac, and a flowering Dogwood tree. The road is visible way in the distance on the left – about 180 yards away.

Another view of the row of flowering trees showing the Eastern Redbud, the lilac, the flowering dogwood, and another redbud in the row. Behind them, growing out from under the large fir tree is another lilac bush.

I wonder if you could propagate that branch?
The variety it’s growing from looks like a hybrid to me, as well. My grandmother had old fashioned lilacs growing all along the edge of the yard on the farm and they were true old-fashioned, one color lilac colored lilacs. Yours has a lighter color along the edge of the flower.
My sister’s oldest son is my godson and I planted a small white lilac in her back yard over 15 years ago on the day he graduated from high school. I saw it in fill bloom when I was there over Memorial Day weekend, it’s full grown and about 10 feet tall, and the blooms smelled heavenly.
Yes it is a hybrid of some kind and that single stem was quite a stark contrast to the rest of the plant. Perhaps we’ll see if it comes back next year, or was just an aberration this year. We can’t remember seeing it in previous years, but perhaps we just never noticed. We have about half a dozen lilacs around the yard, and it does smell good while outside at that time of year. This time of year – not so much – it’s been a terrible (or is that good?) year for the cottonwood trees. Until I mowed on Monday this week our yard almost looked like it had been snowed on, and the garage is full (well it seems like it) of the stuff.