What is the difference between Japa and meditation?

Japa is silent repetition of the Name of the Lord. Meditation is the constant flow of one idea of God. When you repeat Om Namo Narayanaya, it is Japa of the Vishnu Mantra. When you think of the conch, disc, mace and lotus flower in the hands of Vishnu, His ear-rings, the crown on His head, His yellow silken Pitambar, etc., it is meditation. When you think of the attributes of God such as omniscience, omnipotence, etc., it is also meditation.

Swami Sivananda