It’s quite hard to define a word such as meditation. It’s an experience, and surely everyone experiences it differently. A book I came across recently called Steps to Raja Yoga by Swami Atmatattwananda Saraswati has a full chapter devoted to meditation. Here is an excerpt that defines meditation:
Definition of meditation as per the Yoga Darśanam: The sanskrit word for meditation is known as 'Dhyana'. According to Maharşi Patanjali, the founder of Yoga philosophy, meditation is defined as (P.Y.S- 3:2) Tatra Pratyaya Ekat- anata Dhyanam'. Tatra 'there', or in a particular stage. Pratyaya means the content of consciousness or the modifications of the mind. Ekatanata means unbroken stream or uninterrupted thought. When the contents of the mind remain at one point or thought and continue for a long time, the experience is known as meditation. The Sanskrit word Dhyana is also derived from the root word 'Dhyai' which means Chinta' contemplation, or deep thinking in order to achieve the higher level of consciousness. A spontaneous and continued state of the mind focused on any particular process or object is described as meditation.