also: さつきあめ
noun
early-summer rain; rainy season rain
Refers to the intermittent rain typical of the early summer rainy season (tsuyu) in Japan, often with a poetic or literary feel.
五月雨がしとしとと降り続いている。
The early-summer rain is falling gently and persistently.
この時期は五月雨のせいで洗濯物が乾かない。
At this time of year, the laundry won't dry because of the rainy-season rain.
梅雨 is the standard term for the rainy season itself, while 五月雨 specifically refers to the rain that falls during that period, often with a more literary nuance.
五月 (the fifth month of the lunar calendar, roughly corresponding to June) + 雨 (rain). The reading さみだれ is a sound shift from さつきあめ (五月雨).