"Tribulation," according to Jesus, occurs as the result of one's faith in God and is limited to the persecution
of that faith. It is distinct from other causes of persecution (i.e. racism, socio-economic class, etc.) and can
happen at any time.
However, Jesus makes a distinction when speaking of tribulation in the context of the end of human history.
The great tribulation begins when the "abomination of desolation" is "standing in the holy place" and,
according to Daniel (Dan 9:27), signals the last 3-1/2 weeks
of Daniel’s 70th week prophecy, and it ends just before Jesus Christ returns
(Matt 24:29; Mark 13:24)
Because Jesus applies the term "tribulation" exclusively to Believers, He implies that Believers
will be living through this period, and the tribulation ends with the demise of the last genuine Believer.
Believers who die during this period of the Great Tribulation are recognized as "tribulation
saints" (Rev 7:14) which is revealed in the first interlude of
the book of Revelation as those souls released under the altar when Jesus Christ breaks the fifth seal
For deeper study:
What and When is the Tribulation?