Boyd’s Bible Dictionary:

A bushy evergreen, whose flowers, leaves, and berries were much used by Hebrews for perfume, ornament, and spicery

Concise Bible Dictionary:

The well-known tree bearing this name, the myrtus communis. Branches were to be taken from this tree, among others, to make booths when the feast of tabernacles was kept. During the millennium, “instead of the brier shall come up the myrtle tree,” so that it seems to be a type of peace and blessing (Neh. 8:1515And that they should publish and proclaim in all their cities, and in Jerusalem, saying, Go forth unto the mount, and fetch olive branches, and pine branches, and myrtle branches, and palm branches, and branches of thick trees, to make booths, as it is written. (Nehemiah 8:15); Isa. 41:1919I will plant in the wilderness the cedar, the shittah tree, and the myrtle, and the oil tree; I will set in the desert the fir tree, and the pine, and the box tree together: (Isaiah 41:19); Isa. 55:1313Instead of the thorn shall come up the fir tree, and instead of the brier shall come up the myrtle tree: and it shall be to the Lord for a name, for an everlasting sign that shall not be cut off. (Isaiah 55:13)). In Zechariah 1:8,10-118I saw by night, and behold a man riding upon a red horse, and he stood among the myrtle trees that were in the bottom; and behind him were there red horses, speckled, and white. (Zechariah 1:8)
10And the man that stood among the myrtle trees answered and said, These are they whom the Lord hath sent to walk to and fro through the earth. 11And they answered the angel of the Lord that stood among the myrtle trees, and said, We have walked to and fro through the earth, and, behold, all the earth sitteth still, and is at rest. (Zechariah 1:10‑11)
a man (that is, an angel of Jehovah) was seen standing among the myrtle trees, when all the earth was sitting still and was at rest—emblem of the blessing of Jerusalem, for which the angel was interceding. Under the rule of the second Gentile empire, the nations were indifferent to the condition of Jerusalem.