The word "mountainside" is correct and usable in written English.
You can use it to refer to the side of a mountain or hills, usually in a descriptive phrase or figurative usage. For example: The sun slowly set behind the mountainside, painting the sky with brilliant shades of orange and purple.
The troops from 2nd Battalion the Yorkshire Regiment were flown into Arbil, the capital of Iraqi Kurdistan, last week to prepare for a possible deployment of Chinook helicopters on a mission to save families from a little-known religious sect who were stuck on a mountainside.
His white villa, full of fine old paintings, statues and mosaics, clings to the steep mountainside overlooking the Gulf of Salerno.
The Airbus A320, en route from Barcelona to Düsseldorf, had reached cruising altitude before making a sudden and seemingly inexplicable eight-minute descent that ended in the aircraft colliding with a remote mountainside.
The Pank Resort near Sulaymaniyah, the region's second city, is popular with locals as is its mountainside roller coaster.
Out of fuel, and without advanced terrain-detecting radar, the A-10 almost certainly ploughed into a mountainside.
The plant is buried in a mountainside on a military compound near the Iranian city of Qom.In this section Anything more to declare?
These are piped to the mountainside and combined in a snow gun that is powered by electricity.
Juan Luis Londoño, his "superminister" for health and labour, a brilliant economist and capable reformer, was killed when the light aircraft taking him and three aides to a meeting in the provinces crashed into a remote Andean mountainside.
The coca bushes, distinguished by their pale green leaves, have been neatly planted in lines marching up the mountainside.
ALONG the banks of the Mekong in Laos, the forest has been stripped and the mountainside gouged out.
IN THE shimmering air of an arid mountainside, a graceful animal can suddenly speak with a human voice of succour; and a beast that seems to offer its own flesh to a hungry traveller turns out instead to be a provider of water, which is even more desperately needed.
