甌 ao1; 茶甌 dê5 ao1 covered tea cup

甌 ao1 is explained as a xiao3wan3 小碗 a small bowl.  It is further explained as 有蓋的用來泡茶的飲茶具 - which means a cup which has a cover and is used for brewing tea and part of a drinking tea set. The dictionary gave an example of use as 茶甌 dê5 ao1.

On another page of the dictionary, it says 茶碗 cha2wan3 is 茶甌 dê5 ao1: i.e. a teacup is also called a 茶甌 dê5 ao1.  In this particular entry, it writes 帶蓋沏茶碗 - to use a cover to steep tea in a teacup - again referring to gai4wan3 蓋碗 in Mandarin.

From this description, 茶甌 dê5 ao1 is gai4wan3 蓋碗 in Mandarin - a covered cup use for brewing tea (and drinking tea).  In Northern China, people use a gai4wan3 蓋碗 to brew and drink tea.

茶甌 dê5 ao1

In my humble opinion, an ao1 甌 could be mug and it does not necessary have to be covered.  However, a dê5 ao1 茶甌 is a covered tea cup as shown in the above picture.