Ten is a composite number with proper divisors 1, 2 and 5.

10 is the sum of the first three prime numbers ($2 + 3 + 5$) and of the first four integer numbers ($1 + 2 + 3 + 4$).

Also:

• A decagon has 10 sides
• 10 is a triangular number
• 10 is a tetrahedral number
• 10 is a decagonal number
• A cubohemioctahedron has 10 faces
• A pentagonal prism, a pentagrammic prism, a pentagonal antiprism, a pentagrammic antiprism, and a pentagrammic crossed-antiprism have 10 vertices
• A small dodecahemicosahedron and a great dodecahemicosahedron have 10 hexagons
• A pentagonal antiprism, a pentagrammic antiprism, and a pentagrammic crossed-antiprism have 10 triangles.