'baisakh' originates from Sanskrit, specifically the word 'Vaiśākha' (वैशाख), which was the name of a lunar month in classical sources.

'Vaiśākha' passed through Prakrit/Pali and regional pronunciations (e.g. 'Vaishakh'/'Baishakh'), became Nepali 'बैशाख' (baishākh), and was rendered in English as 'baisakh'.

Initially it referred to the lunar month named 'Vaiśākha' in Sanskrit texts; over time it has come to denote the first month of the modern Nepali (Bikram Sambat) calendar, roughly mid-April to mid-May.