The Traditional Chinese Medical (TCM) terms: 經 (jing), 絡 (luo), and 脈 (mai), (along with their combinations: 經絡 (jing luo), 經脈 (jing mai), and 絡脈 (luo mai)) all describe channels or pathways of qi and/or blood that flow through the body. These terms are normally all translated using the same English word: meridian. Unfortunately, the word meridian is a rather poor choice as its original definition (imaginary line of constant longitude on a celestial object) doesn't acurately describe any of the above terms.
The TCM term 經 (jing) refers to the main path of an organ-associated channel. It is used to describe a channel which runs deep in the flesh (ie. cannot be seen), but can be felt (ie. has a pulse). It is said to be a pathway for 氣 (qi).
It was originally analogous to the Western term: artery (from the Greek "aer": air, and "terein": to carry).
Arterial tissue is covered by layers of muscle, which allow it to quickly pump oxygen-rich blood to different organ systems. When death occurs, the body undergoes rigor mortis (the contraction of muscle tissue after death). This expels the arterial blood supply into capillaries and veins. While the body is alive, arteries can be felt as full and pulsing, but when a corpse's arteries are opened up for examination, this process leaves them empty. This led early anatomists to believe that they must have been carrying air.
In TCM theory, 氣 (qi) is understood to be the source of blood's motion. The detectable pulsing of 經 (jing) channels is attributed to qi, which is understood to be the reason bright red blood would spurt or shoot out of a cut jing (artery) in a living animal. Since 經 (jing) channels can be more easily felt than seen, jing diagnosis relies on touch (taking pulses).
The character 經 (jing) is formed by combining the "thread" radical with the character "jing" (course: a pictograph of a river underneath a surface over a scholar), and illustrates threads following a channel or path.
The TCM term 絡 (luo) refers to an alternative branch (or branches) of other channels. It is used to describe a channel which runs just under the surface of the skin (ie. it can be seen), but cannot be felt (ie. has no pulse). It is said to be a pathway for 血 (blood).
It was originally somewhat analogous to the Western term vein (from the Latin "vena": carrier).
Unlike arteries, veins (which are themselves non-muscular) rely on the contraction of the muscle around them to facilitate moving of blood (valves prevent its backflow), thus post-mortem examination revealed vessels full of blood.
In TCM theory, luo vessels primarily housed blood rather than qi, hence the slow drainage of dark blood from a cut vessel. Since they can be seen, but no pulse felt, luo diagnosis relies on sight (colour, shape, size etc...) rather than touch.
The character 絡 (luo) is formed by combining the "thread" radical with the character "ge" (separate: a pictograph of a person in the act of pursuit and speaking at odds), denoting threads joining separate objects.
The TCM term 脈 (mai) can refer to a point, a pulse, or a channel or path. It is often appended to the terms 經 (jing) and/or 絡 (luo) to clarify their meaning.
The character 脈 (mai) is formed by combining the "flesh" radical with the character "pai" (tributary: a pictograph of joining rivers), denoting the joining of tributaries related to (in, under, around, of) flesh.