What is Trigram?

A three-line figure, the building block of hexagrams. There are eight trigrams: ☰ Qian (Heaven), ☷ Kun (Earth), ☳ Zhen (Thunder), ☴ Xun (Wind), ☵ Kan (Water), ☲ Li (Fire), ☶ Gen (Mountain), and ☱ Dui (Lake). Each trigram is associated with a natural phenomenon, a family member, a direction, and a set of qualities defined in the Shuogua Zhuan.

Source: Shuogua Zhuan (說卦傳), Discussion of the Trigrams