Definition of Excusable Neglect
In practice, and particularly with reference to the setting aside of a judgment taken against a party through his “excusable neglect,” this means a failure to take the proper steps at the proper time, not in consequence of the party’s own carelessness, inattention, or willful disregard of the process of the court, but in consequence of some unexpected or unavoidable hindrance or accident, or reliance on the care and vigilance of his counsel or on promises made by the adverse party. As used in rule (e.g. Fed.R.Civil P. 6(b)) authorizing court to permit an act to be done after expiration of the time within which under the rules such act was required to be done, where failure to act was the result of “excusable neglect”, quoted phrase is ordinarily understood to be the act of a reasonably prudent person under the same circumstances. Conlan v. Conlan, Ky., 293 S.W.2d 710, 712.
That's the definition of Excusable Neglect in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.