The word ṣiyām or ṣawm linguistically means to abstain or stop doing something.
In the religion however, the word means to fast: to abstain from food, drink, and intimate relations – with a sincere intention to please Allah (awj) – during all hours of the day. This is based on His command:
Then complete the fast (ṣiyām) until the night. [Sūrah al-Baqarah, 2:187]
Abū Hurayrah (radiAllahu anhu) said that he heard the messenger of Allah (sallah Allahu alayhi wasallam) say that Allah (awj) said:
“Every deed of the Son of Adam is for himself except fasting – it is for me and I will personally reward it.” I swear by the one in whose hand is the soul of Muhammad, the breath in the mouth of a fasting person is better to Allah than the smell of perfume. Muslim (no. 1151)