noun
Formal or bureaucratic term for a period of one year, often used in contracts, plans, and official documents. Equivalent to 一年 but more formal.
See also: 一年
この契約の有効期間は一カ年とする。
The term of this contract shall be one year.
一カ年の計画を立てる。
We make a one-year plan.
一年 is the general word for 'one year' in everyday speech. 一カ年 is a formal, often written, variant used in official contexts.
Compound of 一 (one) + カ年 (counter for years). The カ is a simplified form of 箇, a counter for items, used here to count years.