Angel - Vine's Expository Dictionary of New Testament Words
Angel[ 1,,G32, angelos ]
a messenger" (from angello, "to deliver a message"), sent whether by God or by man or by Satan, "is also used of a guardian or representative in Revelation 1:20, cp. Matthew 18:10; Acts 12:15 (where it is better understood as = 'ghost'), superior to man, Hebrews 2:7; Psalms 8:5, belonging to Heaven, Matthew 24:36; Mark 12:25, and to God, Luke 12:8, and engaged in His service, Psalms 103:20. "Angels" are spirits, Hebrews 1:14, i.e., they have not material bodies as men have; they are either human in form, or can assume the human form when necessary, cp. Luke 24:4, with Luke 24:23, Acts 10:3 with Acts 10:30.
"They are called 'holy' in Mark 8:38, and 'elect,' 1 Timothy 5:21, in contrast with some of their original number, Matthew 25:41, who 'sinned,' 2 Peter 2:4, 'left their proper habitation,' Jude 1:6, oiketerion, a word which occurs again, in the NT, only in 2 Corinthians 5:2. Angels are always spoken of in the masculine gender, the feminine form of the word does not occur."* [* From Notes on Thessalonians, by Hogg and Vine, p. 229.]
Note: Isangelos, "equal to the angels," occurs in Luke 20:36.