An aromantic is someone who does not experience romantic attraction.
An aromantic person is an individual who does not experience romantic love or attraction, although this does not preclude them from feeling other forms of love or attraction, such as platonic love. Keep in mind, aromanticisim is a spectrum, so people may experience different forms of attraction, but still consider themselves to be aromantic.
Aromantics may be interested in relationships without romance involved, such as platonic life partnerships. Platonic, for lack of better definition, is like friendship.
For various thoughts in regards to aromanticism, see the myths.
Aromantic is sometimes abbreviated as "aro". And aromanticism is the noun that refers to aromantics.
There is no official aromantic flag yet, however, there are at least three that have been proposed.