In 2014 I hiked in the Adirondack Mountains, New York, United States of America. The trail passed a sign that had a single word on it: "POSTED". There was no other text on the sign either on the front or on the back. Not understanding what it meant, I ignored it. I camped around 10 metre behind it. When I returned the following day, a fellow hiker commented on it and told me I had camped in an area I wasn't supposed to enter in the first place.
According to this SE answer, in New York state specifically:
It is a defense to this type of trespass that there are not signs posted instructing people to stay off the property. It may also be a defense that any signs posted in the area are not in the proper place to be easily seen by visitors to the property.
Would a sign that only says "POSTED" with no other text be a sign "instructing people to stay off the property"? I wasn't shot and left no trace, so the landowner probably never found out I trespassed, but I'm still wondering about the validity of the sign that is cryptic to foreigners like me, who may not be familiar with the specific context in which the term "POSTED" is used on signs in the USA.