Boyd’s Bible Dictionary:

(God with us). Name of the prophetic child (Isa. 7:14). The Messiah (Matt. 1:23).

Concise Bible Dictionary:

Names of the Messiah prophetically announced, meaning “God with us.” The introduction of this name is remarkable. Ahaz king of Judah, being attacked by Rezin king of Syria, and Pekah king of Israel—and there being also a disposition to form a confederacy to set up the son of Tabeal, and so reject the son of David, which Ahaz was—Isaiah was sent to tell him to be quiet and fear not. Jehovah then told Ahaz to ask for a sign, either in the deep or in the height above; but Ahaz refused to ask; therefore the Lord gave him this sign, “Behold, the virgin shall conceive, and bear a son, and shall call his name Immanuel. Butter and honey shall he eat, that he may know to refuse the evil, and choose the good” (Isa. 7:14-15).
It has been asked, How could this be a sign to Ahaz, seeing that this event did not take place till centuries after? The prophetic announcement of the birth of such a child was the present evidence to faith that whatever combinations men might make, the remnant could count on God: see Isaiah 8:9-10, where they say, “God is with us.” It should be noticed that there are two prophetic children: the one (Shear-jashub) figurative of the remnant, and Immanuel; so the prophecy continues, “For before the child shall know to refuse the evil, and choose the good, the land whose two kings thou fearest shall be forsaken” (as Isa. 7:16 should be translated). In Isaiah 7:3 of this chapter Isaiah had been told, when he went to meet Ahaz, to take his symbolical child Shear-jashub (“the remnant shall return”) with him. And doubtless Isaiah 7:16 refers to Shear-jashub; and before this child could have grown to maturity, Pekah had been killed by Hoshea, and Damascus had been taken and Rezin slain by the king of Assyria (2 Kings 15:30; 2 Kings 16:9).
It may seem strange that there should be no break between Isaiah 7:15 and Isaiah 7:16, as the one refers to Immanuel, and the other to Shear-jashub; but such abrupt transitions are not unusual in prophetic scriptures. When the Lord was in the temple, and speaking of His Father’s house, He was asked for a sign, He said, “Destroy this temple, and in three days I will raise it up,” referring to His body. And in the prophecies the future is often closely associated with what related to passing events. In Isaiah 8:8 it is foretold that the wing of the king of Assyria should fill the breadth of the land—the land of Immanuel—which took place soon after, yet this is a type of the Assyrian’s attacks in the last days.
In the New Testament we get the fulfillment of the above prophecy: Mary the virgin conceived and brought forth her Son. His name was Jesus, and also Emmanuel, “God with us,” showing that He was God, and became man (Matt. 1:23).

Strong’s Dictionary of Hebrew Words:

from 5973 and 410 with a pronominal suffix inserted; with us (is) God; Immanuel, a type name of Isaiah's son
KJV Usage:

Jackson’s Dictionary of Scripture Proper Names:

with us is God

Potts’ Bible Proper Names:

God with us:―name of the prophetic child [EMMANUEL], Isa. 7:14. {Nobiscum Deus}