Both the Bible and Rabbinical sources deal with the Ten Lost Tribes.
There were 12 or 13 Tribes (when Joseph is divided into two) ten of whom separated.
The expression 10 Tribes is found in regard to this separation.
1-Kings (NASB) 11:31
He said to Jeroboam, 'Take for yourself ten pieces; for thus says the LORD, the God of Israel, 'Behold, I will tear the kingdom out of the hand of Solomon and give you Ten Tribes.
1-Kings (NASB) 11:35
'but I will take the kingdom from his son's hand and give it to you, even Ten Tribes.
The Ten Tribes are described as being 'lost.'
Isaiah (HCSB) 27:
13 On that day a great trumpet will be blown, and those lost in the land of Assyria will come, as well as those dispersed in the land of Egypt; and they will worship the LORD at Jerusalem on the holy mountain.
The word translated above as "lost" in Hebrew is "ovdim". This is translated in different ways BUT "lost" is a reasonable rendition and this is how it was understood.
The Ten Tribes had been exiled by the Assyrians and had lost awareness of their ancestry. The term "Lost Ten Tribes" is therefore appropriate.
According to the Commentary of Rashi on Isaiah 27:13, Those who were taken beyond the River by the Assyrians (i.e. the Ten Tribes) and those who were expelled via Mitsrayim-Egypt meaning Judah will be gathered together.
The land of Israel from the Nile to the Euphrates will have the non-Israelites shaken out of it while Judah and Israel will be gathered back into it one by one.
