previous life; past existence
Refers to a life one lived before the current one, often in a Buddhist or reincarnation context. Can also be used more loosely for a former phase of life.
He says he has memories of his previous life.
I wonder what I did in my past life.
前世 (a past life before this one) is easily confused with 来世 (the next life after this one) because both pair with 現世 in Buddhist cosmology but point in opposite time directions.
Written the same as 前世 (ぜんせい, 'a past era'), but read ぜんせ it instead means 'a previous life' in reincarnation.
Compound of 前 (ぜん, 'previous') and 世 (せ, 'world' or 'life'). The reading ぜんせ is the standard on'yomi combination.