An algebra is a vector space over a field which also has a bilinear multiplication
Which distributes over vector addition:
And for it satisfies:
Unit element
The algebra has a unit element if there is an element which acts as a left and right multiplicative identity:
Associative algebra
The algebra is associative if its multiplication is associative:
Examples
is an algebra with unit element
is an algebra with unit element
Any field is an algebra with unit element, but an algebra with unit element is not necessarily a field since it may have nonzero elements that do not have a multiplicative inverse