(1) siyaam in arabic means abstaining; in islam it means abstaining from things that break the fast, from dawn until sunset, having first made the intention (niyyah) to fast.
ruling on fasting

(2) the ummah is agreed that fasting the month of ramadaan is obligatory, the evidence for which is in the qur’aan and sunnah. allaah says (interpretation of the meaning):
“o you who believe! observing al-sawn (the fasting) is prescribed for you as it was prescribed for those before you, that you may become al-muttaqoon (the pious).” [al-baqarah 2:183]

the prophet (peace and blessings of allaah be upon him) said: “islam is built on five [pillars]…” among which he mentioned fasting in ramadaan. (reported by al-bukhaari, al-fath, 1/49).

 whoever breaks the fast during ramadaan without a legitimate excuse has committed a serious major sin, the prophet (peace and blessings of allaah be upon him) said, describing a dream that he had seen: “… until i was at the mountain, where i heard loud voices. i asked, ‘what are these voices?’ they said, ‘this is the howling of the people of hellfire.’ then i was taken [to another place], and i saw people hanging from their hamstrings, with the corners of their mouths torn and dripping with blood. i said, ‘who are these?’ they said, ‘the people who broke their fast before it was the proper time to do so,’ i.e., before the time of iftaar.” (saheeh al-targheeb, 1/420).

al-haafiz al-dhahabi (may allaah have mercy on him) said, “among the believers it is well-established that whoever does not fast in ramadaan without a valid excuse is worse than an adulterer or drunkard; they doubt whether he is even a muslim at all, and they regard him as a heretic and profligate.” shaykh al-islam [ibn taymiyah] (may allaah have mercy on him) said: “if a person does not fast in ramadaan knowing that it is haraam but making it halaal for himself to do so, kill him; and if he does it because he is immoral [but believes it is haraam], then punish him for not fasting.” (majmoo’ al-fataawa, 25/265).