excusable neglect

Definition from Nolo’s Plain-English Law Dictionary

A legitimate excuse for the failure of a party or his or her lawyer to take required action on time (like filing an answer to a complaint). This is usually claimed to set aside a default judgment for failure to answer or otherwise respond within the required time period. Illness, press of business by the lawyer (but not necessarily the defendant), or an understandable oversight by the lawyer's staff ("just blame the secretary") are common excuses which the courts will often accept.

Definition provided by Nolo’s Plain-English Law Dictionary.