入定 is a title that can be defined as Zen contemplation in Japanese or sitting quietly in (Buddhist) meditation in Chinese. It also carries a similar meaning in Korean Hanja. Therefore, this is a universal term for meditation in the context of Buddhism throughout the Orient.

Can also be translated as “Meditatively equipoised” or “enter into meditation by stilling the karmic activities of deed, speech, and thought.”

The original Sanskrit word is samapanna. In Tibetan: snyoms par zhugs pa.

First, let's correct something: The Japanese romanization for this character, “Zen” has penetrated the English language. In English, it's almost always incorrectly used for phrases like “That's so zen.” Nobody says, “That's so meditation” - right? As the title of a sect, this would be like saying, “That's so Baptist!"

禪 by itself just means “meditation.” In that context, it should not be confined to use by any one religion or sect.

Regardless of the dictionary definition, more often than not, this character is associated with Buddhism. And here is one of the main reasons:
Zen is used as the title of a branch of Mahayana Buddhism, which strongly emphasizes meditation practice.
However, it should be noted that Buddhism came from India, and “Chan Buddhism” evolved and developed in medieval China. The Chinese character “Chan” was eventually pronounced as “Zen” in Japanese. Chan Buddhists in China have much in common with Zen Buddhists in Japan.

More about the history of Zen Buddhism here.

Please also note that the Japanese Kanji character for Zen has evolved a little in Japan, and the two boxes (kou) that you see at the top of the right side of the character have been replaced by three dots with tails. The original character would still be generally understood and recognized in Japanese (it's considered an ancient version in Japan) but if you want the specifically modern Japanese version, please click on the zen Kanji to the right. Technically, there is no difference between the Tensho and Reisho versions of Zen since they are ancient character styles that existed long before Japan had a written language.

There is also an alternate/shorthand/simplified Chinese version, which has two dots or tails above the right-side radical. This version is also popular for calligraphy in China. If you want this version, just click the character to the right.

Further notes: Zen is just one of seven sects of Buddhism practiced in Japan. The others are 律 Ritsu (or Risshū), 法相 Hossō, 論 Sanron 華嚴 Kegon, 天台 Tendai, and 眞言 Shingon.

參禪 is a title that speaks of reaching an understanding (of Zen or the world). It also means “to practice meditation.” The two concepts lead you to the idea that meditation leads to understanding. 參禪 is pretty deep, so you can do your research or decide what this means for you.

This can also be defined in a more complex way as “thoroughly penetrating with meditative insight.”