


Cardinal  one  
Ordinal  1st (first) 

Numeral system  unary  
Factorization  1  
Divisors  1  
Roman numeral  I  
Roman numeral (unicode)  Ⅰ, ⅰ  
Greek prefix  mono /haplo  
Latin prefix  uni  
Binary  1_{2}  
Ternary  1_{3}  
Quaternary  1_{4}  
Quinary  1_{5}  
Senary  1_{6}  
Octal  1_{8}  
Duodecimal  1_{12}  
Hexadecimal  1_{16}  
Vigesimal  1_{20}  
Base 36  1_{36}  
Greek numeral  α'  
Persian  ١  یک  
Arabic  ١  
Urdu  
Ge'ez  ፩  
Bengali & Assamese  ১  
Chinese numeral  一，弌，壹  
Korean  일, 하나  
Devanāgarī  १  
Telugu  ೧  
Tamil  ௧  
Kannada  ೧  
Hebrew  א (alef)  
Khmer  ១  
Thai  ๑  
Malayalam  ൧ 
One (1) is a natural number after zero and before two. It represents a single item. A human typically has one {head, nose, mouth, and navel (bellybutton)}. The Roman numeral is I.
Mathematics
In mathematics, one is also the only number with this special fact: Any number multiplied by 1 equals that number. Then, the number 1 divides any number but it is not divided by any except itself. The number 1 divides any number to give that same number but it cannot be divided by any other number to give one as the answer. Let n be any number. Then n/1 = n but 1/n is never 1 (when n is not 1). Example: 7/1 = 7 but 1/7 is not 1 but 0.142857 ...
Any number divided by itself always gives 1 as the answer. n/n = 1 (when n is not 0). Example: 7/7 = 1.
0.999... is a repeating decimal that is mathematically equal to 1. Many proofs have been made to show this is correct.
Computer science
The number one is important for computer science because it is, together with zero, a part of the binary numeral system. In machine code and in lots of programming languages one means true. Zero means false.
Other meanings
 In Germany and Austria, one is the grade for "very good". It is the best grade of six possible grades in Germany and the best of five possible grades in Austria. In the Netherlands, one is the lowest grade, and ten the highest and best. In Poland, one is also the lowest grade, but the highest is six.
 In numerology, the number one is a symbol for everything (unity), the beginning, and God.