The ability to focus the attention on one thing without distraction. The power of conscious concentration (directed attention) is developed in all genuine spiritual traditions. In Hindu and Buddhist systems, concentration (dharana, shamatha, etc) is noted as a middle step towards real meditation.
"Without wise concentration of thought, the experience of the truth is impossible." —Samael Aun Weor, Spiritual Power of Sound
"...concentration is the first and foremost thing a sadhaka or aspirant should acquire in the spiritual path." —Swami Sivananda
"If you focus the rays of the sun through a lens, they can burn cotton or a piece of paper; but, the scattered rays cannot do this act. If you want to talk to a man at a distance, you make a funnel of your hand and speak. The sound-waves are collected at one point and then directed towards the man. He can hear your speech very clearly. The water is converted into steam and the steam is concentrated at a point. The railway engine moves. All these are instances of concentrated waves. Even so, if you collect the dissipated rays of the mind and focus them at a point, you will have wonderful concentration. The concentrated mind will serve as a potent searchlight to find out the treasures of the soul and attain the supreme wealth of Atman (the Self), eternal bliss, immortality and perennial joy." —Swami Sivananda
See also: Frequent Questions about Meditation