A curve which touches each member of a family of curves and which is touched at each point by some member of the family is called the envelope of the family of curves.
Example: Consider the family of straight lines
y = mx + a/m ,
where m is the parameter and a is some given constant.
We know that each member of the family touches the parabola y2 = 4ax. Also the parabola y2 = 4ax has at every point the tangent which is of the form y = mx + a/m . Hence y2 = 4ax is the envelope of the given family of lines.