From BiblePortal Wikipedia

Smith's Bible Dictionary [1]

Sid'dim. (Field, Plain). The Vale Of Siddim . A place named only in one passage of Genesis -  Genesis 14:3;  Genesis 14:8;  Genesis 14:10. It was one of that class of valleys, which the Hebrews designated by the word, emek . This term appears to have been assigned to a broad, flattish tract, sometimes of considerable width, enclosed on each side by a definite range of hills. It was, so far, a suitable spot for the combat between the four and five kings,  Genesis 14:8; but it contained a multitude of bitumen pits sufficient materially to affect the issue of the battle.

In this valley. The kings of the five allied cities of Sodom, Gomorrah, Admah, Zeboim and Bela seem to have awaited the approach of the invaders. It is, therefore, probable that it was in the neighborhood of the "plain or circle of Jordan" in which those cities stood. If we could venture, as some have done, to interpret the latter clause of  Genesis 14:3, "which is near," or "which is at, or by, the Salt Sea," then we might agree with Dr. Robinson and others in identifying the valley of Siddim, with the enclosed plain which intervenes between the south end of the lake, and the range of heights, which terminate the Ghor , and commence the Wady Arabah . But the original of the passage seems to imply that the Salt Sea covers the actual space formerly occupied by the vale of Siddim. See Sea, The Salt.

People's Dictionary of the Bible [2]

Siddim (Sĭd'Dim ), A Depression. The vale in which were "slime pits," near the plain whereon stood the cities of Sodom and Gomorrah.  Genesis 14:3;  Genesis 14:10;  Genesis 19:28. See Sodom.

Holman Bible Dictionary [3]

 Genesis 14:1

American Tract Society Bible Dictionary [4]

See Sea 3.