(Science: medicine) hay fever, nasal catarrh accompanied with fever, and sometimes with paroxysms of dyspnoea, to which some persons are subject in the spring and summer seasons. It has been attributed to the effluvium from hay, and to the pollen of certain plants. It is also called hay asthma, hay cold, and rose fever. Hay knife, a sharp instrument used in cutting hay out of a stack or mow. Hay press, a press for baling loose hay. Hay tea, the juice of hay extracted by boiling, used as food for cattle, etc. Hay tedder, a machine for spreading and turning newmown hay.