Have you ever read an article about acupuncture and the word "meridian" was used? Sometimes articles explain the meaning of meridians well, but other times they're described as vague "energy channels" or "pathways for the flow of Qi". These explanations aren't wrong, but unless you've been through acupuncture school these definitions can be difficult to grasp.
According the Chinese Acupuncture and Moxibustion by Cheng Xinnong, which is one of the fundamental required texts for beginning acupuncture students, meridians are "responsible for the circulation of Qi and Blood and distributed both interiorly and exteriorly across the body." Basically what this means is that Qi (also known as oxygen, nitrous oxide and various vital minerals) along with Blood circulate throughout the body via the meridians.
We know that Blood circulates via blood vessels throughout the body, but are blood vessels meridians? Likely not. The reason is because acupuncture points are located along meridians, and when an acupuncture needle is inserted into the body, it goes into connective tissue (which sometimes gives a heavy, mild achy sensation signaling the arrival of Qi) rather than blood vessels (which would create pain and excessive bruising). Therefore, meridians are likely connective tissue lines that run alongside blood vessels, which makes sense because needling a tight acupuncture point will help release constricted connective tissue that might be slightly impeding the circulation of Qi and Blood. Keeping these lines of connective tissue healthy will promote good circulation in the body, which ultimately can improve overall health.
A breakthrough 2002 study revealed the relationship between acupuncture points, meridians and connective tissue. Below is a very cool image from that research. An acupuncture needles was inserted into a rat and images were taken. What's observed is the acupuncture needle is grasping the connective tissue when the needle is twisted.