The Dhammacakkappavattana Sutta is the Buddha's first discourse after he reached Enlightenment. In this sutta, the Buddha discusses the Middle Way, The Noble Eightfold Middle Path and The Four Noble Truths.
It is located in the Samyutta Nikaya, chapter 56, written in abbreviation as SN 56. 11.
Dhamma (Pāli) or "Dharma" (Sanskrit) can mean a variety of things depending on its context; in this context, it refers to the Buddha's teachings or his "truth" that leads to ones liberation from suffering. "Chakka" (Pāli) or "chakra" (Sanskrit) can be translated as "wheel."
The "dhammacakka," which can be translated as "Dhamma-Wheel," is a Buddhist symbol referring to Buddha's teaching of the path to enlightenment. "Pavattana" (Pāli) can be translated as "turning" or "rolling" or "setting in motion."
So the full definition of the First Sermon can be called, "Setting in Motion the Wheel of the Dhamma."
In this discourse, the Buddha addresses five monks who attended to him just prior to his Enlightenment. The Buddha cautions the monks against pursuing either of two extremes: worldly sensual pleasures or painful self-mortification. The Buddha refers to the path that avoids these extremes as the "Middle Way" (majjhimā patipadā).
The Buddha then states that the middle way he awakened to involves pursuing a "Noble Eightfold Path" (ariyo atthangiko maggo) that includes "right" (sammā) understanding, thought, speech, action, livelihood, effort, mindfulness and concentration. This path leads to vision, knowledge, peace, enlightenment and Nibbana.