What is confounded?

Some articles on confounded:

Fractional Factorial Design - Resolution
... Resolution Ability Example II Not useful main effects are confounded with other main effects 22 − 1 with defining relation I = AB III Estimate main effects, but these may be confounded with two-factor ...
Glossary Of Experimental Design - Glossary
... the treatment effect and the blocking effect are said to be confounded ... resolution" describes the degree to which the estimated main-effects are aliased (or confounded) with estimated higher-order interactions (2-level interactions, 3-lev ... If some main effects are confounded with some 2-level interactions, the resolution is 3 ...