Please add a photo. It may or may not be a lilac if the leaves look different - it might be a tree seedling.

Thanks for the reply.

I haven't been able to get a good angle on it, but I will go out and try again when it stops raining. Our lilac has been trained like a tree, and this other branch is growing out of the trunk--so not a seedling, I don't think.

May just be a sucker as well. Lilacs produce both root suckers as well as branch suckers and these often display very healthy, vigorous growth. But a photo would confirm.