What is Lesion?

Legal Definition
A lesion is any abnormal damage or change in the tissue of an organism, usually caused by disease or trauma. Lesion is derived from the Latin word laesio meaning injury. Lesions may occur in plants as well as animals.
-- Wikipedia
Legal Definition
Contracts. In the civil law this term is used to signify the injury suffered, in consequence of inequality of situation, by one who does not receive a full equivalent for what he gives in a commutative contract.

2. The remedy given for this injury, is founded on its being the effect of implied error or imposition; for in every commutative contract, equivalents are supposed to be given and received. Louis. Code, 1854. Persons of full age, however, are not allowed in point of law to object to their agreements as being injurious, unless the injury be excessive. Poth. Oblig. P. 1, c. 1, s. 1, art. 3, §4. But minors are admitted to restitution, not only against any excessive inequality, but against any inequality whatever. Poth. Oblig. P. 1, c. 1, s. 1, art. 3, §5; Louis. Code, art. 1858.

3. Courts of chancery relieve upon terms of redemption and set aside contracts entered into by expectant heirs dealing for their expectancies, on the ground of mere inadequacy of price. 1 Vern. 167; 2 Cox, 80; 2 Cas. in Ch. 136; 1 Vern. 141; 2 Vern. 121; 2 Freem. 111; 2 Vent. 359; 2 Vern. 14; 2 Rep. in Ch. 396; 1 P. W. 312; 1 Bro. C. C. 7; 3 P. Wms. 393, n.; 2 Atk. 133; 2 Ves. 125; 1 Atk. 301; 1 Wils. 286; 1 Wils. 320; 1 Bro. P. 6. ed. Toml. 198; 1 Bro. C. C. 1; 16 Ves. 512; Sugd. on Vend. 231, n. k.; 1 Ball & B. 330; Wightw. 25; 3 Ves. & Bea. 117; 2 Swanst. R. 147, n.; Fonb. notes to the Treatise of Equity, B, 1, c. 2, s. 9. A contract cannot stand where the party has availed himself of a confidential situation, in order to obtain some selfish advantage. Note to Crowe v. Ballard. 1 Ves. jun. 125; 1 Hov. Supp. 66, 7. Note to Wharton v. May. 5 Ves. 27; 1 Hov. Supp. 378. See Catching bargain; Fraud; Sale.
-- Bouviers Law Dictionary
Legal Definition
Fr. Damage; injury; detriment. Kelham. A term of the Scotch law. In the civil law. The injury suffered by one who does not recoive a full equivalent for what he gives in a commutative contract. Civil Code La. art. 1860. Inequality in contracts. Poth. Obi., no. 33. In medical jurisprudence. Any change in the structure of an organ due to injury or disease, whether apparent or diagnosed as the cause of a functional irregularity or disturbance,'
-- Black's Law Dictionary
Legal Definition
One’s loss in receiving less than his full equivalent under a commutative contract. See 109 La. 948, 34 South. 34. See, also, Commutative contract.
-- Ballentine's Law Dictionary