Satya, or truth, is one of the yamas Patanjali refers to in the ancient yogic text, the Yoga Sutra. But telling the truth may not be as easy as it sounds. So what does telling the truth mean? To some yogis it means speaking with the intention of being truthful, given that what we call the "truth" is filtered through individual experiences and beliefs about the world. But when we speak with that intention, I have a better chance of not harming others.
Another aspect of satya has to do with inner truth or integrity, a deeper and more internal practice. Honesty is what we do when others are around and might judge our actions or words, but to have integrity is to act in an honest manner when others are not around and will never know about our actions.
In Sanskrit, sat means the eternal, unchanging truth beyond all knowing; ya is the activating suffix which means "do it." So satya means "actively expressing and being in harmony with the ultimate truth." In this state we cannot lie or act untruthful, because we are unified with pure truth itself.
From http://www.everydayhealth.com (under the yoga section).