Air & Water: Air, water, earth, and fire are the four known elements. What does the fifth element stand for?

Heart.

As per ancient Hindu philosophy its Akash. Akash is an extremely fascinating element which in modern terms could relate to vacuum or dark matter or aether. Note the subtle difference between air (wind) and Akasha. It amazes me that ancients could even think of a possibility of Akasha without physically experiencing it. From wikipedia: "The pancha mahabhuta, or "five great elements", of Hinduism are kshiti or bhūmi (earth), ap or jala (water), tejas oragni (fire), marut or pavan (air or wind), vyom or shunya or akash) (aether or void). Hindus believe that all of creation, including the human body, is made up of these five essential elements and that upon death, the human body dissolves into these five elements of nature, thereby balancing the cycle of nature. According to one of the principal texts of Hindu philosophy, the Tattwa Kaumudi authored by Vacaspati in the 9th century A.D., the Creator used akasha (ether), the most "subtle" element, to create the other four traditional elements; each element created is in turn used to create the next element, each less subtle than the last. The five elements are associated with the five senses, and act as the gross medium for the experience of sensations. The basest element, earth, created using all the other elements, can be perceived by all five senses - hearing, touch, sight, taste, and smell. The next higher element, water, has no odor but can be heard, felt, seen and tasted. Next comes fire, which can be heard, felt and seen. Air can be heard and felt. "Akasha" (ether) is the medium of sound (vibration) but is inaccessible to all other senses." "Akasha (or Akash, Ākāśa, आकाश) is the Sanskrit word meaning "aether" in both its elemental and metaphysical senses." "its main characteristic is Shabda (sound). In Sanskrit the word means "space", the very first element in creation. In Hindi, Marathi and Gujarati, and many other Indian languages, the meaning of Akasha has been accepted as sky. Akasha is space in the Jain conception of the cosmos. It falls into the Ajiva category, divided into two parts: Loakasa (the part occupied by the material world) andAloakasa (the space beyond it which is absolutely void and empty). In Loakasa the universe forms only a part. Akasha is that which gives space and makes room for the existence of all extended substances." "In Ayurvedic terms, Ether (Akasha) means “space” or “emptiness” and is the empty space between atoms. As we know from quantum physics, matter is much less solid than it appears, and is mostly composed of empty space. Ether also corresponds to what many call the undifferentiated cosmic force, the unified field, or the one mind. One characteristic of Ether is sound or vibration; its main characteristic is lack of resistance. It is the lightest of the elements. " http://en.wikipedia.org/wiki/Classical_element#Classical_elements_in_Hinduism http://en.wikipedia.org/wiki/Akasha http://yogasynergy.com.au/main/5-elements

Most people agree: it is not a simple, 'normal' element. The Chinese have five (Metal Wood Water Fire Earth), but these are 'normal' elements. The Fifth element is usually transcendental, spiritual, different. Metaphysical, even; the Greeks called it Quintessence (fifth-element); some think of it as spirit, or Aether, the ever-pervasive meta-substance which exists in all matter; the Japanese think of it as Mu, or emptiness, void, without; Captain Planet calls it Heart; I like the interpretation that Milla Jovovich with Orange Hair/Love is the Fifth Element. Technically, technically it's Boron (B).

The premise of the question is incorrect. There are a lot more than five elements. 118 are known of currently, and scientists suspect that there are more: http://en.wikipedia.org/wiki/Periodic_table_of_elements

As this was posted in science and thus turned up in my feed I'll just say 1. Quarks 2. Leptons 3. Bosons To get it to five, one could split up leptons into tauonic, muonic and electronic leptons. Of course some could argue only quarks and leptons count as matter, giving a total of 12 "elements" of matter if every type is listed separately.