A hydrophobic residue from lubricants, skin oils, food oils, machinery, sealants, or other oily materials transferred to glass.
GlassRenu · Educational illustration
Meaning and boundaries
The source and formulation may be unknown, and some apparent oil marks are silicone, plasticizer, adhesive, or coating effects rather than ordinary grease.
Why it matters in glass restoration
Oily residue can smear, trap abrasive particles, change wetting, contaminate tools, and interfere with polishing or protective treatments.