A more thorough explanation:

Definition: Fosterage is the act of taking care of someone else's child, or the act of promoting or encouraging something.

Examples:

• When a child's parents are unable to take care of them, they may be placed in fosterage with another family.
• The fosterage of a new business idea can help it grow and succeed.

The first example illustrates the first and second definitions of fosterage, as it involves the act of caring for another's child and the entrusting of a child to another. The second example illustrates the fourth definition of fosterage, as it involves the act of promoting or encouraging something, in this case a business idea.