Atma(soul) + Karaka(the one that’s responsible) = atma-karaka. The atmakaraka in ones chart would indicate the sin that was performed in ones previous birth. The very reason we were born in this janmam will be clearly shown by analysing the atma-karaka. There are many ‘karakas(significators) in vedic astrology, specifically written in the Jaimini sutras.
Technically, there’s two atma-karakas, one is the natural , which is always the Sun and the other is chara-atmakaraka which shows the personality of jeevatma (your natural traits). The chara-atmakaraka can be easily identified as the highest degree (longitude) in a chart.
Om Tat Sat