mediti

Two senses live here, and they are far enough apart to be worth separating. The first is the practice: sitting still, quieting the mind, the thing a monk or a yoga teacher does, and it takes no object at all. Li meditas duonhoron ĉiutage. The second is ordinary deep reflection about a subject, and it takes pri: mediti pri la estonteco, to ponder the future. English "meditate" covers both too, but the first sense has swallowed the second in modern usage, so learners often assume mediti is only the cushion-and-incense word. It is not.

Against pensi, the difference is duration and depth rather than kind. Pensi is what your head does all day, including deciding what to have for lunch. Mediti is sustained, deliberate and slow, and you cannot mediti for two seconds. Related nearby: pripensi is to think something over, which sits neatly between the two, and it is the word to reach for when "consider" is what you mean.

The nouns are medito for a single reflection and meditado for the practice as an ongoing thing, which is the one you want on a poster for a class. A meditejo is a room set aside for it.