海军 means "navy" and refers to the branch of a country's military that operates at sea. It is used in both modern and historical military contexts.
他的爷爷曾经是一名海军军官。
tā de yéye céngjīng shì yì míng hǎijūn jūnguān.
His grandfather was once a naval officer.
这个国家的海军实力非常强大。
zhège guójiā de hǎijūn shílì fēicháng qiángdà.
This country's naval strength is very powerful.
海军基地位于城市的南边。
hǎijūn jīdì wèiyú chéngshì de nánbiān.
The naval base is located to the south of the city.