Hyssop is the plant named in the English Bible as an agent of ritual purification, though the exact species intended by the Hebrew ezov is uncertain and may have been a marjoram or savory rather than the garden hyssop, Hyssopus officinalis, later given the name. Its biblical role is unmistakable: at the first Passover the Israelites are told to take a bunch of hyssop and strike the blood on the doorposts, and the fiftieth Psalm prays, purge me with hyssop and I shall be clean. It is thus, above all, the herb of cleansing from sin and of the sparing of the firstborn.
Modern scholarship has largely resolved, though not unanimously, the old puzzle of which plant the Hebrew ezov names: since true hyssop, Hyssopus officinalis, does not grow wild in the Holy Land and is native instead to southern Europe, most botanists and biblical scholars now favour Syrian oregano, Origanum syriacum, the aromatic herb still gathered across the Levant today as za'atar, whose soft, liquid-holding leaves and rocky habitat fit every ceremonial use the text describes. Medieval Jewish commentators including Maimonides and Saadia Gaon had already identified ezov with this same herb centuries before modern botany took up the question, so the garden hyssop that later gave the plant its English name may be, strictly, a case of a translation choosing the wrong familiar plant for an unfamiliar one.