I'm with Flyerdan, hydraulic oil, other than that, i'm at a loss.I tweaked your image so it's visible in the forum, hopefully someone will be along with a definitive answer, but I'd guess that it means hydraulic oil (obviously) perhaps under pressure as noted by the piston.
What machine is this on? If it's brand specific it might help to move this to the proper forum where it will be seen by people familiar with them.
Found a couple symbols about New Holland warning lights that look somewhat similar. It could have something to do with charge pressure. Still help to know what it's on.I'm with Flyerdan, hydraulic oil, other than that, i'm at a loss.