What is Breach Of Trust?

Legal Definition
The wilful misappropriation, by a trustee, of a thing which had been lawfully delivered to him in confidence.

2. The distinction between larceny and a breach of trust is to be found chiefly in the terms or way in which the thing was taken originally into the party's possession; and the rule seems to be, that whenever the article is obtained upon a fair contract, not for a mere temporary purpose, or by one who is in the. employment of the deliverer, then the subsequent misappropriation is to be considered as an act of breach of trust. This rule is, however, subject to many nice distinctions. 15 S. & R. 93, 97. It has been adjudged that when the owner of goods parts with the possession for a particular purpose, and the person who receives them avowedly for that purpose, has at the time a fraudulent intention to make use of the possession as the weans of converting the goods to his own use, and does so convert them, it is larceny; but if the owner partwith the property, although fraudulent means have been used to obtain it, the, act of conversion is not larceny. Id. Alis. Princ. c. 12, p. 354.

3. In the Year Book, 21 H. VII. 14, the distinction is thus stated: Pigot. If I deliver a jewel or money to my servant to keep, and he flees or goes from me with the jewel, is it felony ? Cutler said, Yes : for so long as he is with me or in my house, that which I have delivered to him is adjudged to be in my possession; as my butler, who has my plate in keeping, if he flees with it, it is felony. Same law; if he who keeps my horse goes away with, him: The reason is, they are always in my possession. But if I deliver a horse to my servant to ride to market or the fair and he flee with him, it is no felony; for e comes lawfully to the possession of the horse by delivery. And so it is, if I give him a jewel to carry to London, or to pay one, or to buy a thing, and he flee with it, it is not felony : for it is out of my possession, and he comes lawfully to it. Pigot. It can well be: for the master in these cases has an action against him, viz., Detinue, or Account. See this point fully discussed in Stamf. P. C. lib. 1; Larceny, c. 15, p. 25. Also, 13 Ed. IV. fo. 9; 52 H. III. 7; 21 H. VII. 15.
-- Bouviers Law Dictionary
Legal Definition
Any act done by a trustee contrary to the terms of his trust, or in excess of his authority and to the detriment of the trust; or the wrongful omission by a trustee of any act required of him by the terms of the trust. Also the wrongful misappropriation by a trustee of any fund or property which had been lawfully committed to him in a fiduciary character.
-- Black's Law Dictionary
Legal Definition
The violation of the duties of his trust by one acting as a fiduciary.
-- Ballentine's Law Dictionary