Who is the Angel of the Lord?

       In the Old Testament or, as the Hebrews refer to, the Tanakh. There are
many passages that refer to an Unknown Angel. This Angel is referred to as
'an Angel of the Lord'.

In my Hebrew Class, i am currently enrolled, which is affiliated with Hebrew
University(eteacher). This passage has become an enigma to me. Especially,
significant because my Instructor pointed out that it could or may be translated
differently. This is how it is worded in my course text i.e. Tanakh translation.

 וַיִּקְרָא אֵלָיו מַלְאַךְ יְהוָה, מִן-הַשָּׁמַיִם, וַיֹּאמֶר, אַבְרָהָם אַבְרָהָם; 
 וַיֹּאמֶר, הִנֵּנִי. 

And an angel of the Lord called to him from the heavens, and he said,"Abraham,
Abraham!" And he said,"Here I am." ~ Genesis 22:11

My Transliteration

vayiqera  e(la)yn  mal(e)ad (adonai) min-hasasamaim vayomer ab(e)raham
ab(e)raham vayomer hin(e)niy 
~My Apologies to Hebrew Language Speakers i probably sound like a Texan 
from the deep south trying to speak HRH Queens English
  
אֵלָיו מַלְאַךְ
 e(la)yn  mal(e)ad
Loosely translated 'an angel' is contextually out of place, but rather the  
 (la) in context with Lord predicates a definite article 'the' as apposed to 'an'

Why is this important? Simply because contextually it causes confusion. It is 
translated in the Tanakh as 'an angel' because in the context of the passage the angel
suddenly appears in the story and it doesn't seem to be any specific angel. However,
it is grammatically correct to refer to the translation as 'the angel of the Lord'.
Christian Old Testaments do refer to this and many other similar passages translated
as 'the angel of the Lord'