Hawaiians refer to it as mana, the Greeks called it pneuma, the Japanese call it ki, yogis know it as prana, devout Christians might think of it as the Holy Ghost, martial artists know it is energy, and my 10-year-old son insists it the ineffable force that lends atoms awareness of themselves. Spelled qi (pronounced 'chee') in pinyin Chinese (otherwise ch'i), it is the root currency of Traditional Chinese Medicine, that energy that TCM practitioners manipulate to fight disease and assure health.
In fact, TCM recognizes several different kinds of qi. There is the primordial qi that arises at the union of sperm and egg, the daily qi nourished by what we eat and drink and how we move, the surface qi (wei qi) that guards us against invading pathogens, and the particular qi unique to different organs and systems (liver qi, heart qi, etc.). As a concept in the Chinese lexicon, however, qi has more than merely medical meaning. It appears in common parlance--your qi looks good today--and runs like a meandering stream through Chinese literature, arts, music and philosophy.
Recently, while filming a series of documentaries on Eastern healing modalities, I have been trying to zero in on a tangible, practical definition of qi, and to express it in scientific terms. This is tough for two reasons: first, since we don't know exactly what it is, designing studies to test for it, ferret it out, or quantify it presents certain problems of investigative design, and second because the concept itself is the product of a characteristically Eastern (and perhaps unique) way of looking at the world, which sees systems rather than components and attempts to cohere rather than dissect the matter and forces in the known world.
To understand this distinction better, think of the creature we call "snake." In pursuit of this reptile, Western science sends researchers into the field to collect specimens (early in my career I was a herpetologist and did exactly this kind of work), make careful notes as to the collection location, "pickle" the "specimens" by dropping them in formaldehyde, and then take them back to the lab. Once there, the dead snake would be cut open for clues about its biology. Noting anatomical structures would lead to cladistics (the establishment of relationships between this species of snake and others) and inferences about its behavior, such as noticing a dead mouse in its digestive tract and thereby presuming a favoring of rodents. The point, of course, is that to understand things, Western science takes them apart.
The Chinese approach--informed and suffused with Taoism, a nondual philosophy seeing an integration of everything--is different. In seeking to understand not only the snake but also "snakeness," the Taoist would be more likely to sit in the forest and watch the snake live its life. She might observe how the snake travels, how it climbs, how it subdues its preferred prey, how it basks in the sun to gain energy, what it does in the rain, how it selects a mate (and what reproduction looks like), whether it lays eggs or gives birth to live young (yes, some snakes do) and what predators pursue it. Then and only then might a specimen be cut open to ascertain the appearance of its lungs or the morphology of its kidneys. Both approaches are complementary, and indeed necessary for deep understanding of things--and indeed modern biologists in China would certainly proceed along the familiar, Western path of inquiry--but the concept of qi yields its secrets reluctantly to our Western, reductionist style.
What we can say for sure is that qi is nonmaterial, and therefore, as TCM suggests, falls under the umbrella of "energy." If we think of it as "life force" we can see that it is likely an aggregate of ideas we Westerners would tend to separate. We could say it probably requires circulation, both of lymphatic fluid and blood, and may also include heat, bioelectricity (the electric potential across cell membranes), sound and even photonic energy (light). In fact a qi-investigating scientist at the University of California measured an increase in the stream of photons issuing from an acupuncture point in my palm after I practiced tai chi as compared with before that exercise.
We might also wish to think of qi as a sort of biological information contained in DNA, this despite the idea that in a broader sense even inanimate objects may possess it. In fact, practitioners of Chinese feng shui (geomancy) see qi as earth energy whose flow, when properly regulated and encouraged, makes for a happy, fertile home, and when ignored or misused results in a living or working space that can lead to stagnant creativity, illness and misery. Qi might also be considered to include the vibration of strings or subatomic particles, and therefore a feature of the quantum universe.
Debating the existence of qi seems a more popular pursuit than attempting to define what the term means. Remember, in seeking a definition, that from the Western perspective it is either a word for something we have not yet discovered, or for an aggregate of phenomena we currently recognize. Perhaps the best we can do is to decide whether or not the term is useful. If you have engaged in a mind/body practice for any length of time, you have probably experienced qi in a direct way that obviates the need for scientific validation. You may construe it simply as the energy of enthusiasm, "get up and go," or a familiar tingling in your hands. You may also recognize qi as a quality that steers you clear of certain people and draws you to others. You may know it as the quality of a certain food or foods to uplift you, as contrasted to another food that slows and weighs you down.
Knowing qi, feeling it, sensing it, you can learn what techniques and choices enhance it and allow it to flow freely (stagnant qi is seen as a source of disease) and you may find this sensitivity helps you stay healthy and enjoy life more. At very least, entertaining the idea of qi introduces the notion of as-yet-undefined forces into the nuts-and-bolts way we look at the world--a view limited both by our senses and by the accepted truths of the day--and thereby deepens our experience of what it is to be alive.