1.
one (thing)
The number one when counting things using native Japanese numbers.
2.
one year old
Age of one year (informal counting).
一つ is the native Japanese number 'one,' used for counting general items and as part of many set expressions.
NATIVE COUNTING (1-10):
COMMON PATTERNS:
AGE:
Used informally for age one: 子供が一つになった (the child turned one). This native system (ひとつ, ふたつ...) is used for young children's ages up to about nine.
USAGE:
This native counting system is used for general items when no specific counter applies, or when the counter is unknown.