Yoga is derived from the Sanskrit root yuj, to join, to unite, to attach. The English word yoke is cognate with the Sanskrit word yoga.
We can think of yoga as the joining of the atma with the paramatma, the soul with God.
There are numerous means of joining with God:
through action, karma-yoga;
through knowledge, jnana-yoga;
through devotion, bhakti-yoga;
through meditation, dhyana-yoga, etc.
Yoga has many other meaning.