EDIT Definition from »Other Dictionary Data«, Unihan Database one; a, an; alone

 Unihan Database 

EDIT Use This character is used for the number 1, but not for the figure 1 in multi-digit numbers.
Example: you could use it for »group 1 wins« or »just 1 more day« but not for »just 111 more days« or »fabrication number 1431-215«.

 jsw 

EDIT Chinese numbers 1 to 10 are scattered over various codepoints:

1 一 U+4E00
2 二 U+4E8C
3 三 U+4E09
4 四 U+56DB
5 五 U+4E94
6 六 U+516D
7 七 U+4E03
8 八 U+516B
9 九 U+4E5D
10 十 U+5341

 jsw