The Greek adjective/noun aitios means 'responsible for,' 'the cause of,' or 'the one who is guilty.' It can be used positively (the 'author' of something good) or negatively (the 'cause' of harm, guilt). In Hebrews 5:9, it is used of Christ as the 'source' or 'author' of eternal salvation.
The most theologically weighty use of aitios is in Hebrews 5:9, where Jesus, 'having been made perfect, became the source (aitios) of eternal salvation to all who obey him.' The word that elsewhere denotes guilt or responsibility here describes Christ as the originating cause of salvation. Just as a person can be the responsible agent of harm, Christ is the responsible agent — the Author — of eternal life. This is the flip side of Adam's causal role in sin and death (Romans 5:12). Where Adam was the aitios of death, Christ is the aitios of life. Luke uses the word in Acts 19:40 of the 'cause' for the Ephesian riot — another legal usage — showing how the same term bridges ordinary human accountability and divine redemptive agency.