According to Wikipedia, angels are defined as:
“An angel is a supernatural being or spirit found in various religions and mythologies. In Zoroastrianism and Abrahamic religions they are often depicted as benevolent celestial beings who act as intermediaries between Heaven and Earth, or as guardian spirits or a guiding influence. Other roles of angels include protecting and guiding human beings, and carrying out God‘s tasks. The term “angel” has also been expanded to various notions of spirits found in many other religious traditions. The theological study of angels is known as “angelology”.
Through our experience, however, we have come to understand that perhaps angels are indeed benevolent beings/ascended souls, that have lived many lives, and experienced all experience – so they are able to exist outside of the body, and incarnate as they choose (not as what is chosen for them). The definition perhaps has its validity, but may be limited by the consciousness of those who have experienced angelic encounters, and what their thoughts projected them to appear as.
If we are living in a universe that has been formed through frequency, and vibration – perhaps angels exist on a higher frequency that we are not all able to tune to, but exists nonetheless.