Lilacs are subject to several different leaf diseases. Most of which are not serious under normal conditions.
Some leaf diseases are not easily distinguishable to leaf damage caused by physiological factors. We need to know when were they planted, is this the first year you have seen this, what fertilizers, weed killers might have been used around the plant, when did you first see this. Leaf diseases that occur at the end of the season frequently are not a problem. For now I would clean up fallen leaves and take some samples from the bush and send them to your local extension service.
With the weather we had this year many plants are showing problems that are not usually seen. Most of these will not be problems with normal weather.

What IpmMan said. If it is an actual pathogen causing the damage it might be halo blight, Pseudomonas syringae. It can express itself in different ways (leaf blight, bud blast, cankers, etc.). Can also affect plums, cherries and pears.
hortster

Here is a link that might be useful: Halo blight

It's tough to diagnose a problem when only one leaf is in the image.

That said, to me it looks like acute drought damage because the tissues between the veins are brown. That's the last place the water gets to when a shortage is sudden, as during a heat wave or similar circumstances.

A little history about the lilac would help, too. For instance, was it planted recently? If so, the roots aren't yet extensive enough to support the growth.

Please post an overall view & fill in a few details about the lilac's history.

Rather then a disease your Lilac may have Lilac Borers.

Here is a link that might be useful: About Lilac Borer