What was the Sanskrit word for union?

Answer 1

The word 'yoga' is correct for union if meant in the esoteric sense. i.e, the union of the Individual Soul(jIvAtma) with Universal Soul (paramAtma/God) is meant.

If worker's union, trade union is what is meant then sa~NghaH(सङ्घ:) would be ideal qualified noun. the qualifiers would have to be kArmika(कार्मिक) or vyApAra/vANijya (व्यापार/वाणिज्य) would be better.

Answer 2

The Sanskrit word for union is "Yoga." It refers to the union of the individual soul with the universal consciousness, as well as the physical practice of yoga postures and breathing techniques.