The continuation of twelve concentrations or twelve Dharanas on any object or on the appearance of your Deity or Guru with closed eyes in; Trukuti is called Dhyana or MEDITATION. You chant mantras while meditating. Twelve concentrations equal 144 seconds. 

There are two types of meditation 

SUGUNA Meditation : Meditation on god or Goddess or Guru with form.
NIRGUNA Meditation : Meditation without any form. Meditation acts a bridge to establish a connection between the individual soul (Jivatma) and the supreme soul (Paramathma).

 
Meditation is the flow of continuous thought on one idea to get one pointedness of mind ;
Concentration, Meditation and Samadhi are Internal Sadhanas.