An acanthus is a herbaceaous plant or shrub of the genus Acanthus. They have spiny leaves and tend to be big and noticeable. The kind of plant you'd describe as handsome, architectural or impressive, rather than pretty.
Talking of architecture, acanthus leaves are often the inspiration for stylized representations of plant life, often created from stone, used to decorate buildings, especially the tops of columns. Those can be called acanthus too.