Yoga has come to mean many things, but a small amount of study reveals the central meaning. Yoga is the nondual state, advaita, and the process of attaining it.
The nondual state is associated with a cessation of mental fluctuations, the settling of arguments, seeing things from all points of view, peaceful inclusion. It often comes along with big picture perspectives, looking past the concerns of your physical self, an absence of inner chatter and neurosis.
Sage Vasiṣṭha states that all of reality – the visible and the invisible, the past and the future – everything that ever was or ever will be exists upon the one substrate of absolute consciousness.
“Just as the uncarved image is forever present in the block, the apparent world – whether you regard it as real or unreal – is inherent in the absolute”
Yoga Vasiṣṭha III.10
So, yoga is the nondual state, when you have an experience of being “one with many” where everything and everyone is contained in one large moment. That sounds nice.
Now, the term yoga also refers to practices that lead us to the state of nonduality. Such as behavioural guidelines, physical techniques that balance strength and sensitivity, all the way up to acts of concentration and devotion performed in consciousness.
These practices are meant to give us greater contact with the nondual state. Sometimes immediately, but most of the time it is an accumulation process. Regular yoga practice means that serendipitous moments of higher perspective are more frequent in your life.