Literally, Sanskrit: dhyan means application of attention to the purpose of the aspirant. You consider the purpose with full application of attention or focus. It is the sharpness and sustenence of one-pointed focus which is the key to unlocking the door to the union. How long must you be able to sustain the one-ponued attention? For as long as an idea keeps popping in your head. You can certainly meet any goal you want to achieve when you are able to sustain one-pointed focus on your goal until you have exhaused all ideas on it and your mind goes silent.
Thus total silence of the seeking mind is Yoga. When an aspirant is able to sustain one-pointed focus on any goal to the extent of totally exhausting all possible ideas on it silencing the mind, she/he is able to achieve union with the ultimate purpose of life.