Skepticism

Skepticism is a Hellenistic school of philosophy. It holds that one should refrain from making truth claims, and avoid the postulation of final truths.

This is not necessarily quite the same as claiming that truth is impossible (but some thinkers argue abouy it). It is often also used to cover the position that there is no such thing as certainty in human knowledge.

Gorgias claimed that nothing exists; or, if something does exist, then it cannot be known; or if something does exist and can be known, it cannot be communicated. 

Socrates claimed that he knew one and only one thing: that he knew nothing. Thus, rather than making assertions or opinions, he set about questioning people who claimed to have knowledge.

Pyrrho advocated suspending judgment on all matters, arguing that certainty is unattainable and that embracing doubt leads to tranquility (ataraxia).