What is Acquiescence?

Legal Definition
In law, acquiescence occurs when a person knowingly stands by without raising any objection to the infringement of their rights, while someone else unknowingly and without malice aforethought makes a claim on their rights. Consequently, the person whose rights are infringed loses the ability to make a claim against the infringer, or succeed in an injunction suit due to the infringer's conduct. The term is most generally a kind of "permission" given by silence or passiveness.
-- Wikipedia
Legal Definition
Contracts. The consent which is impliedly given by one or both parties, to a proposition, a clause, a condition, a judgment, or to any act whatever.

2. When a party is bound to elect between a paramount right and a testamentary disposition, his acquiescence in a state of things which indicates an election, when he was aware of his rights will be prima facie evidence of such election. Vide 2 Ves. Jr. 371; 12 Ves. 136 1 Ves. Jr. 335; 3 P. Wms. 315. 2 Rop. Leg. 439.

3. The acts of acquiescence which constitute an implied election, must be decided rather by the circumstances of each case than by any general principle. 1 Swanst. R. 382, note, and the numerous cases there cited.

4. Acquiescence in the acts of an agent, or one who has assumed that character, will, be equivalent to an express authority. 2 Bouv. Inst. n. 1309; Kent, Com. 478; Story on Eq. 255; 4 W. C. C. R. 559; 6 Miss. R. 193; 1 John. Cas. 110; 2 John. Cas. 424 Liv. on Ag. 45; Paley on, Ag. by Lloyd, 41 Pet. R. 69, 81; 12 John. R. 300; 3 Cowen's R. 281; 3 Pick. R. 495, 505; 4 Mason's R. 296. Acquiescence differs from assent. (q. v.)
-- Bouviers Law Dictionary
Legal Definition
Acquiescence is where a person who knows that he is entitled to impeach a transaction or enforce a right neglects to do so for such a length of time that, under the circumstances of the case, the other party may fairly infer that he has waived or abandoned his right. Scott v. Jackson, 89 Cal. 258, 26 Pac. 898; Lowndes v. Wicks, 69 Conn. 15, 36 Atl. 1072; Norfolk & W. R. Co. v. Perdue, 40 W. Va, 442, 21 S. El 755; Pence v. Langdon, 99 U. S. 578, 25 In Ed. 420.

Acquiescence and laches are cognate but not equivalent terms. The former is a submission to, or resting satisfied with, an existing state of things, while laches implies a neglect to do that which the party ought to do for his own benefit or protection. Hence laches may be evidence of acquiescence. Laches imports a merely passive assent, while acquiescence implies active assent. Lux v. Haggin, 69 Cal. 255, 10 Pac. 678; Kenyon v. National Life Ass'n, 39 App. Div. 276, 57 N. Y. Supp. 60; Johnson-Brinkman Commission Co. v. Missouri Pac. R. Co., 126 Mo. 345, 28 S. W. 870, 26 L. R. A. 840, 47 Am. St. Ren. 675.
-- Black's Law Dictionary
Legal Definition
Implied assent.
-- Ballentine's Law Dictionary