From BiblePortal Wikipedia

Hawker's Poor Man's Concordance And Dictionary [1]

The Urim and Thummim are supposed to have been the precious stones worn by the high priest upon his breast-plate, when going into the temple, and before the mercy seat. The meaning is supposed to be lights and perfections. But farther than these explanations, the multitude of commentators have not advanced. It is the happiness of the church in Jesus, however, to look to all the shadows of the law, through the medium of the gospel, and to discover every thing is the former as ministering but to the latter. So that when Aaron was thus adorned and went in before the propitiatory, he represented our Almighty Aaron, who was, and is himself, both the light and the life, the perfection, and the glory of all his redeemed. Hence when Moses in his dying prediction of the children of Israel, declared that JEHOVAH'S Urim and Thummim should be with his Holy One, none could be alluded to but the Lord Jesus Christ, and in him, the whole tendency of both, had their accomplishment.

People's Dictionary of the Bible [2]

Urim (Û'Rim ), and Thummim (Thŭm'-Mim ), Light and Perfection. Denote some part of the high priest's apparel. When Aaron was arrayed, Moses himself put the Urim and Thummin into the breastplate.  Exodus 28:13-30;  Leviticus 8:8. Nowhere in Scripture are the Urim and Thummim described, and we do not know what they really were.

King James Dictionary [3]

U'RIM, n. Heb. The Urim and Thummim, among the Israelites, signify lights and perfections. These were a kind of ornament belonging to the habit of the high priest, in virtue of which he gave oracular answers to the people but what they were has not been satisfactorily ascertained.

Webster's Dictionary [4]

(n.) A part or decoration of the breastplate of the high priest among the ancient Jews, by which Jehovah revealed his will on certain occasions. Its nature has been the subject of conflicting conjectures.