The Entmoot was a rare gathering and meeting of Ents.
In Fangorn Forest, these moots were always held in Derndingle, a bowl-like valley far south of Treebeard's home. Since the Ents' language was so descriptive and extensively slow, an Entmoot would endure unbearably long.
The Entmoot held in The Lord of the Rings was held from February 30 to March 2, 3019, of which the subject was the decision whether the Ents should go to war against Saruman. It lasted from noon on the first day till afternoon on the third day, and by Ents, it was considered quick for an Entmoot. 
A moot is a regular gathering of people having a common interest, hence an Entmoot is a gathering of Ents.