The Meaning of al-Niyyah (Intention)
An important aspect that must be discussed is the meaning of the word ‘intention’ (niyyah).
Shaykh Saalih al-Sadlaan discusses the linguistic roots of the word al-niyyah and then has a passage that describes the meaning of the word in this context:
“The real purpose of intention:
Some say that it is seeking something. Some say that it means seriousness in one’s seeking.
For example, ibn Mas’ood once said: “Whoever intends this worldly life will not be able to achieve it,” that is, one who seriously seeks it. Some say that it means to intend something in the heart. Others say that it means to resolve in the heart.
Some say that it comes from the word al-nawa which means to distance as if the intention for something implies seeking something with purpose and resolve that one cannot reach by his limbs and apparent actions due to its distance from him. Hence, this intention becomes the means by which he achieves it.
Ibn Qayyim al-Jawziyyah defined intention as the knowledge of the doer of what he is doing and what is the purpose behind [this action]. He stated that the intelligent, voluntary actor does not do anything without first conceiving it & wanting it.
This is the reality of intention.
Intention is not something external to the conceptualization of the person and his purpose to do it.
Al-Suyutee said: “Intention describes the driving force in the heart towards what the person sees to be in conformity with what he wants, of either bringing about good or putting off harm, both present and future.” 
The meaning of niyyah can be described in English as: endeavour, intention, purpose, resolution, goal, aim, resolve, determination & so forth.
It is not just a thought that comes to one’s mind but is a determination, want & aspiration to do something.
Therefore, if one has the intention or niyyah to do something it means that he will do it as long as there is nothing preventing him or as long as he does not change his intention.
For example, if one intends to fast on Monday & then Monday comes & there is nothing to prevent him from fasting, yet he does not fast, this means that he did not truly have the intention once Monday actually arrived.
This is the linguistic meaning of the word niyyah & there is no evidence that the Sharee’ah meaning of the word is different. However, the jurists use it in a slightly different sense. They refer to it as the intent in the heart that must accompany & precede any act of worship.”
Footnote:  Sadlaan, al-Niyyah, vol. 1, pp. 98-99.
SOURCE: Jamaal al-Din M. Zarabozo , Commentary on the 40 Hadith of al-Nawawi, p.104