In the Design of Experiments a Balanced Design (Balanced Experiment) is a factorial design in which each factor is run the same number of times at the high and low levels.
This two factor design shown below is balanced because both factors ('A' and 'B') are run an equal number of times (twice) at both levels:
Treatment

A

B

AB + C

1

1

1

+1

2

+1

1

1

3

1

+1

1

4

+1

+1

+1

Most factorial designs are balanced, unbalanced designs are only used in exceptional circumstances.
