What is this on my lilac?
The brown bump on your lilac bush is an egg case (ootheca) of the praying mantis. These are beneficial insects in the garden. They prey upon many other insects and provide a natural form of pest control. The egg case won't do harm to your lilac. You can just leave it there, or you could cut out the branch with the egg case and lay it in a different area of your yard.
Here is more information about the praying mantis.
Thank you! I will just leave it there.