والصومُ هو: الإمساكُ عن الأكلِ، والشربِ، والجماعِ نهارًا، مع النيةِ.
Translation: Fasting is abstaining from eating, drinking, and intercourse during the day, together with an intention.
The definition does not equate fasting with merely going without food. Abstention from eating, drinking, and intercourse through the prescribed day must be accompanied by an intention.
Someone who happens not to eat or drink for unrelated reasons has not necessarily performed a legal fast. The detailed intention rules appeared earlier; this line shows why intention belongs in the very definition.