A tense formative is an infix that indicates a change in tense. For example, the κα in λελυκαμεν is a tense formative indicating the perfect tense. Other tense formatives are (the more common are bolded):
future active tense σ
Liquid future active εσ (BBG 20.11)
first aorist active σα
first aorist active liquid α
aorist passive θη
future passive θησ
perfect active κα
second perfect active α (BBG 25.14)
perfect active participle κ