The aorist is one of the past tenses of the Greek verb, and denotes a simple past occurrence, unlike the imperfect, which denotes incomplete or progressive action. In the passive voice, it is the aorist of the passive verb, which has a participle, called the aorist passive participle.
On the Wikipedia page: Aorist (Ancient Greek) - Wikipedia, the free encyclopedia
The verb λύω, passive λύομαι, has an aorist form ἐλύθην and a participle λυθείς.
See also the second aorist forms.