The word for fasting in Islamic literature can appear in one of two forms - either الصوم al-sawm, or الصيام al-siyaam. So what is the difference between them, and can they be used interchangeably?
Both the word الصوم al-sawm and الصيام al-siyaam are verbal noun forms from the same root - ص و م s w m, the basic meaning of which indicates the meaning of abstention. But Allah uses them in very different ways in the Qur’an.
In Surah al-Baqarah (verse 183), Allah uses the word صيام siyaam when He says,
يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ كُتِبَ عَلَيْكُمُ ٱلصِّيَامُ كَمَا كُتِبَ عَلَى ٱلَّذِينَ مِن قَبْلِكُمْ
O you who believe! Fasting is prescribed for you—as it was for those before you
In this instance, the word صيام siyaam is being used to indicate a physical abstention (which we refer to as fasting) from food, drink, and marital relations. And thus we find that every single time Allah refers to fasting, the act of worship, in the Qur’an, the word صيام siyaam has been used. Although the word صوم sawm has been used in the hadith to refer to fasting, there is only one time that it has been used in the Qur’an, and here it refers to another kind of abstention - that of the tongue from talking. As Allah says in Surah Maryam (verse 26),
إِنِّى نَذَرْتُ لِلرَّحْمَـٰنِ صَوْمًۭا فَلَنْ أُكَلِّمَ ٱلْيَوْمَ إِنسِيًّۭا
I have vowed an abstention [of speech] to the Most Compassionate, so I am not talking to anyone today.
Scholars of the language and tafsir have speculated as to why this may be - the word صوم sawm is shorter and so is better suited to the meaning of silence; the word صوم sawm and صمت silence are similar in sound and pattern; the word صيام siyaam is longer and so it better suited to the meaning of fasting as there are more things we abstain from - but what is clear is that the language of the Qur’an is precise and perfect.
Not a single letter in the Qur’an is random, nor can it ever be replaced with another.