In the phrases "lyric tenor" or "lyric baritone" what does the word "lyric" signify?

As you
probably already know, musicians categorize the human voice according to range using terms like
bass, baritone, tenor, countertenor, contralto, and soprano. According to the Baker's Student
Encyclopedia of Music available on the tenor has a typical range of "two octaves,
from the C below middle C to the C above (c0-c2)"  and a baritone is "the medium-range
male voice, lower than the tenor and higher than the bass, with a compass from A to about
f1." According to European classical tradition, voice types can be classified even further
than just range, using the "weight" of a voice.

Think about
different tenors you may have heard. Some of them have a darker, heartier, earthier, heaviness
to their tone. Others seem lighter, fluid, and soaring. These two tenors may have the same
range, but they have different vocal weights. Wikipedia calls vocal weight "the perceived
"lightness" or "heaviness" of a singing voice." The article goes on to
state: 

 Lighter voices are often associated with the term
lyric and are usually brighter and more agile; heavier voices are
often associated with the term dramatic and are usually powerful,
rich, and darker.

Lyric voices are those with a lighter
vocal weight. Keep in mind that this doesn't neccesarily mean that lyric sopranos or lyric
baritones can sing higher that others in their vocal range, but they sound
"lighter." Lyric voices may not have the power of their dramatic counterparts, but
when it comes to long, fast runs of notes in their range, they make it sound angelic and
effortless.