Shaykh ibnu Uthaymeen said:

Fasting linguistically means: الإمساك

(To refrain and abstain from something)

This meaning is found in Allah’s saying:

{And if you see any human being, say: verily i have vowed a fast for the Most Merciful (Allaah) so i shall not speak to anyone today} [Maryam #26]

Meaning: I (Maryam) have vowed to refrain from speaking so i will not speak to anyone today.

Religiously it means: A form of worship for Allah by refraining from that which nullifies the fast from the second fajr (dawn/daybreak) to maghrib (sunset).

Fataawaa ibnu uthaymeen p421