두음법칙
Initial Consonants ㄹ/ ㄴ Rule

Initial ㄹ/ ㄴ appearing in Sino-Korean words are kept in the North. In the South, in Sino-Korean words that begin with ㄹ and are followed by the vowel sound [i] or the semi-vowel sound [j] (i.e., when ㄹ is followed by one of ㅣ, ㅑ, ㅕ, ㅖ, ㅛ and ㅠ), ㄹ is replaced by ㅇ; when this ㄹ is followed by other vowels it is replaced by ㄴ. In the North, the initial ㄹ is kept. For example,

North Korean	South Korean	English meaning
리념	이념	ideology
력사	역사	history
래일	내일	tomorrow
로동	노동	labor, work

Similarly, in Sino-Korean words that begin with ㄴ and are followed by the vowel sound [i] or the semi-vowel sound [j] (i.e., when ㄴ is followed by one of ㅣ,  ㅕ, ㅛ, and ㅠ), in the South this ㄴ is replaced by ㅇ, whereas this remains unchanged in the North:

North Korean	South Korean	English meaning
니승	이승	this life, this world
녀자	여자	woman

These are thus pronounced as written in the North as ㄴ |n| and ㄹ |l|. However, even in the South, sometimes in order to disambiguate the surnames 유 (柳) and 임 (林) from 유 (兪 [ju]) and 임 (任), the former may be written or pronounced as 류 (柳) and 림 (林).

- Excerpt from Wikipedia -