A green plant with long stems, leaves like feathers and no flowers. (Cambridge Dictionary)
A fern is a plant that has long stems with feathery leaves and no flowers. There are many types of fern. (Collins Dictionary)
A flowerless plant which has feathery or leafy fronds and reproduces by spores released from the undersides of the fronds. Ferns have a vascular system for the transport of water and nutrients. (Oxford Dictionaries)
Fern. Collins Dictionary.