The fifth house is the fifth of the twelve houses of the horoscope. It governs creativity, self-expression, pleasure, play, romance, and children, the things a person brings forth and delights in, the joyful and generative side of life. It is a succedent house, of moderate strength, following the angular fourth and associated with the fixed, consolidating phase of its quadrant. These are the interpretive conventions of the astrological tradition for this division of the chart, in which the fifth house describes what flows out of a person for the sake of enjoyment and creation, from art and romance to the raising of children; the older tradition also connected it with good fortune and was well disposed toward planets placed there, counting it among the more benefic houses of the wheel.
The Hellenistic astrologers who fixed the twelve-place scheme, traditional already in Vettius Valens around 175 CE and set out systematically by Paulus Alexandrinus in 378, called the fifth house Agathe Tyche, Good Fortune, and held that Venus has her joy there, one of seven places across the wheel assigned to a planet's particular ease. The naming explains why the fifth was reckoned among the most benefic of the twelve houses long before children, romance, or creativity became its standard modern glosses: it was first and foremost a place of luck, the house whose planets were read as bringing good things with comparatively little struggle, and Venus's joy there ties pleasure and fortune together at the root of the house's character rather than as a later elaboration on top of it.
Sources A History of Western AstrologyNicholas Campion with Introduction to Astrology (Eisagogika)Paulus Alexandrinus