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Cosham ( or ) is a northern suburb of Portsmouth lying within the city boundary but off Portsea Island. It is mentioned in the Domesday Book of 1086 along with Drayton and Wymering (mainland) and Bocheland (Buckland), Frodington (Fratton) and Copenore (Copnor) on the island.
The name is of Saxon origin (shown by the -ham suffix) and means "Cossa's homestead". Originally pronounced , since the latter half of the 20th century has become more widely used. Until the 1920s it was a separate small village surrounded by fields (including on the north end of Portsea Island).
Extensive suburban growth then expanded around the village and both east and west along the slopes of Portsdown Hill. It has been for many years a local route centre as a pinch point for buses travelling in and out of Portsmouth and offers three railway routes to London. Cosham railway station was until 1935 the terminus for City trams and trolleybuses from the south and Portsdown and Horndean Light Railway trams to the north. The High Street is a significant local shopping centre. Few traces of the original village now remain; the oldest houses (Chalk Cottage of 1777 and Mile Stone Cottages of 1793) were demolished in the 1960s and replaced by a car park, but the old milepost showing mileage to London, Petersfield and Portsmouth remains. The interior of St Philip's Church (1938) in Highbury is cited as a fine example of Ninian Comper's work. Indeed, England's 1000 Best Parish Churches (by Simon Jenkins) regards St Philip's as the only parish church within Portsmouth worth visiting on architectural merit.
Cosham is also home to the UK headquarters of IBM UK Ltd. The site known as 'North Harbour' was built in the 1970s, the location being selected due to the job cuts by the Royal Navy during the time.
The Queen Alexandra Hospital is situated to the North of the town.
The main campus of Highbury College is situated to the south east of main town.