The Heart (心, pinyin: xīn) is one of the zàng organs stipulated by Traditional Chinese Medicine (TCM). It is a functionally defined entity and not equivalent to the anatomical organ of the same name.
In the context of the zang-fu concept
As a zàng, the Heart is considered to be a yin organ. Its associated yang organ is the Small Intestine. Both Heart and Small Intestine are attributed to the Fire element.
Regarding its stipulated functions, the Heart
The Heart's function is said to be strongest between 11am and 1pm. Disturbed function of the Heart typically presents as palpitations, arrhythmia, insomnia, dream disturbed sleep, poor memory, restlessness, or even delirium.
References
Heart (Chinese medicine) Wikipedia(Text) CC BY-SA