The term “Jacob’s trouble” is found in Jeremiah 30:7.

Jer 30:4  Now these are the words that the Lord spoke concerning Israel and Judah.
Jer 30:5  “For thus says the LORD: ‘We have heard a voice of trembling, Of fear, and not of peace.
Jer 30:6  Ask now, and see, Whether a man is ever in labor with child? So why do I see every man with his hands on his loins Like a woman in labor, And all faces turned pale?
Jer 30:7  Alas! For that day is great, So that none is like it; And it is the time of Jacob’s trouble, But he shall be saved out of it.

Jacob’s trouble is a time of trouble that will come on the descendants of the patriarch Jacob.  Jacob had twelve sons who later became the nations of Israel and Judah.  As Jeremiah 30:4 makes clear, Jacob’s trouble is a time of trouble for the nations of Israel and Judah.  See “Who are Britain and the USA in Bible prophecy” for information on who are the modern-day descendants of Israel and Judah.

As Jeremiah 30:7 states Jacob’s trouble is a time of trouble so great there is none like it. 

This same language is also used speaking of the great tribulation at the time of the end.

Mat 24:21  For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be.

There cannot be two separate events each so great there is none like them.  Jacob’s trouble and the great tribulation must therefore be the same event.

The great tribulation lasts about 3½ years.  It is the last 3½ years before Jesus Christ returns.  See “How long does the Great Tribulation last?”  The great tribulation impacts all nations, but its impact on Israel and Judah is described as Jacob’s trouble.

Conclusion

Jacob’s trouble is the great tribulation at the time of the end, in particular that part which impacts the modern-day descendants of Israel and Judah.