An algebra, A, on a set X satsfies

• empty set in A, X in A
• B in A => comp(B) in A (comp(B) = X/B)
• C,B in A => C union B in A

A sigma algebra, A, on a set X satisfies:

• empty set in A, X in A
• B in A => comp(B) in A (comp(B) = X/B)
• {Ai} 1<=i<infiniti), Ai in A foralli => Union(i=1,infiniti,Ai) in A

Examples:
A={empty set, X}
A=borel sigma algebrea=all open sets

Note that intersection is defined.
A intersct B = Comp(Comp(A) union Comp(B))