The cope is the most versatile of the Christian vestments, and it is defined by a negative: it is what is worn when the service is not the Eucharist.
A processional cloak
The cope is a long semicircular cloak fastened across the chest with an ornamental clasp called the morse, carrying a vestigial hood at the back that survives from its origin as an outdoor garment. It is worn for the offices that are not the Mass, for processions, benedictions, solemn Vespers, and for occasions such as coronations and consecrations, and it may be worn by a bishop, a priest, and in some traditions by cantors and lay ministers, which makes it far less restricted than the chasuble.
Colour and use
Like the other outer vestments the cope follows the colours of the liturgical season, and the grandest examples are among the most elaborate textiles the church possesses, since the garment is large and displayed in procession. Its versatility is the point: because it belongs to the offices rather than to the Eucharist, and because it is not tied to the order of priesthood in the way the chasuble is, it is the vestment most often seen worn by many ministers at once in a great ceremony.