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
The term of this contract shall be one year.
We make a one-year plan.
一年 is the everyday word for 'one year', whereas 一カ年 is a stiffer, formal variant with the same meaning used in official reports and contracts.
Commonly used in official documents and contracts. The small カ is a counter marker.
Variant with small ヶ, also common in formal writing.
Variant with small ヵ, less common but still seen.
Full kanji form, rarely used in modern writing.
Hiragana variant, sometimes used in less formal 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.