Definition

Definition of prayer times
FAJR starts with the dawn or morning twilight. Fajr ends just before sunrise.
ZUHR begins after midday when the trailing limb of the sun has passed the meridian. For convenience, many published prayer timetables add five minutes to mid-day (zawal) to obtain the start of Zuhr. Zuhr ends at the start of Asr time.
The timing of ASR depends on the length of the shadow cast by an object. According to the Shafi school of jurisprudence, Asr begins when the length of the shadow of an object exceeds the length of the object. According to the Hanafi school of jurisprudence, Asr begins when the length of the shadow exceeds TWICE the length of the object. In both cases, the minimum length of shadow (which occurs when the sun passes the meridian) is subtracted from the length of the shadow before comparing it with the length of the object.
MAGHRIB begins at sunset and ends at the start of isha.
ISHA starts after dusk when the evening twilight disappears.