When the effect of one factor depends on the presence of another factor, outcomes are said to reflect a (n) ?

When the effect of one factor depends on the presence of another factor, outcomes are said to reflect an Interaction.

Explanation:

This is a basic association scheme. First of all, when two different phenomenons are related. They are considered to have a determining effect on each one. One of them becomes a dependant variable and the other an independent variable. This relationship is called "interaction" because it is the way they relate to each other. It can be positive or negative and it is still an interaction.

When the effect on a dependent variable (Y) of one factor (X), depends on the presence of another factor (Z), the outcomes are reflecting the existence of an interaction.

An interaction occurs when there is a regression with more than one explanatory variable, and the simultaneous effect of two of them on Y is not additive. Therefore it is necessary to include both variables in the model accompanied by an interaction term that should be implemented as an extra regressor.

To provide extra insight, when interactions happen between X and Z, the presence of X moderates the effect of Z and viceversa.