The abbreviation myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.6 teraseconds. 

Myr is in common use where the term is often written, such as in Earth science and cosmology. Myr is seen with "mya", "million years ago". Together they make a reference system, one to a quantity, the other to a particular place in a year numbering system that is "time before the present". 

Myr is deprecated in geology, but in astronomy "myr" is standard. Where "myr" "is" seen in geology it is usually "Myr" (a unit of mega-years). In astronomy it is usually "MYR" (million years). 

In geology the debate of the millennia concerns the use of "myr" remains open concerning "the use of "Myr" plus "Mya"" versus "using "Mya" only". In either case the term "Ma" is used in geology literature conforming to ISO 31-1 (now ISO 80000-3) and NIST 811 recommended practices. Traditional style geology literature is written The "ago" is implied, so that any such year number "X Ma" between 66 and 145 is "Cretaceous", for good reason. But the counter argument is that having "myr" for a duration and "Mya" for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a "milliyear" (a thousandth of a year, or 8 hours). On this side of the debate, one avoids "myr" and simply adds "ago" explicitly (or adds "BP"), as in In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".
is myr roughly the same as mya in amount?
what does mya stand for?
and myr?
how many teraseconds does it represent?
do they use it in astronomy?
how about in geology?
is there a debate between myr and mya usage?
which one is favored?
is myr avoided?
how do they do that?
which iso does ma conform to?
what NIST recommended practice does it conform to?
yes
Myr is seen with "mya", "million years ago"
million years ago
 "mya", "million years ago"
million years
myr, "million years"
31.6
"million years", is a unit of a quantity of (i.e. ) years, or 31.6 teraseconds
yes
Myr is deprecated in geology, but in astronomy "myr" is standard
no
Myr is deprecated in geology
yes
In geology the debate of the millennia concerns the use of "myr" remains open
the term "Ma"
In either case the term "Ma" is used in geology literature
yes
On this side of the debate, one avoids "myr" 
one avoids "myr" and simply adds "ago" explicitly
On this side of the debate, one avoids "myr" and simply adds "ago" explicitly (or adds "BP")
ISO 31-1 (now ISO 80000-3)
"Ma" is used in geology literature conforming to ISO 31-1 (now ISO 80000-3)
NIST 811
 ISO 31-1 (now ISO 80000-3) and NIST 811 recommended practices.