Lewis & Short

instantĭa, ae, f. [insto].

  1. I. Lit., a being near or close upon, hence presence: futura quorum consequenti tempore vera erit instantia, ea vera dicimus, Cic. Fat. 12, 27 B. and K.: infestus dicitur ab instantia atque imminentia fraudis, the immediate and threatening character of the injury, Nigid. ap. Gell. 9, 12, 6.
  2. II. Trop., steadiness, constancy, perseverance; force, vehemence in speaking; earnestness in supplication, importunity, urgency (post-Aug.): quid est enim, quod haec instantia non possit efficere? Plin. Ep. 3, 5, 18: oratio maxime vi, amaritudine, instantiā placet, id. ib. 5, 8, 10; opp. tarditas, Sol. 2, 46: repetendi debiti instantia, Dig. 32, 1, 32: sed instantia Byrrhaenae perfecit, ut vellet, App. M. 2, p. 123, 38: pertinaci negare instantia, Amm. 28, 1, 19.