oil spot

Short definition from Wiktionary

Noun

1. 1
   A spot consisting of oil: A soiled spot on clothing, a tablecloth, etc., where some oil was absorbed; a similar spot on a driveway, puddle, etc., where some oil is lying.
2. 2
   A spot consisting of oil: An oil film over an area of the ocean after an oil spill.
3. 3
   One of the yellow spots, often circular in shape, on the upper leaf surface of grapevines in the early stages of downy mildew (caused by Plasmopara viticola infection).