Peninsular Malaysia is also known as West Malaysia (Malaysia Barat) or Malaya (Tanah Melayu). The term Peninsular Malaysia is used more often than West Malaysia (to avoid the idea that West and East Malaysia are separate countries like West Germany and East Germany) or Malaya (which is now becoming obsolescent due to its connotations of the British colonial era). Nonetheless, all three terms are correct, and the older term "Malaya" can still be found in many institutional titles, e.g. the High Court of Malaya, the University of Malaya, Malayan Railway, etc., as well as in legal contexts in the phrase States of Malaya (Negeri-negeri Tanah Melayu). Note that until 1946, the term Malaya usually included Singapore.
The distinction between West and East Malaysia is significant beyond the sphere of geography, because as well as having a different court structure, the Eastern states have more autonomy than the original States of Malaya, controlling e.g. immigration from the peninsula.
WestMalaysia is bordered by Thailand to the north, Singapore to the south, South China Sea to the east, and Straits of Malacca to the west.
East Malaysia shares its southern border with Indonesia, and is cradled by the South China Sea to the west and north, and the Sulu Sea to the northeast.
Four-fifths of the population resides in WestMalaysia.