Eternal subordination of the Son ought to be (but is not always) distinguished from the heresy of subordinationism.

Subordinationism has been defined as,

‘A doctrine that assigns an inferiority of being, status, or role to the Son or Holy Spirit within the Trinity.’ (R. C. and C. C. Kroeger, “Subordinationism,” in Evangelical Dictionary of Theology, 1st and 2nd editions)

This, however, is a little too broad, because (setting aside the word ‘inferiority’) it implies that any kind of subordination is to be regarded as heresy.

Similarly, in the Dictionary of the Christian Church (eds. F.L. Cross and E.A. Livingstone) subordinationism is defined as

teaching about the Godhead which regards either the Son as subordinate to the Father or the Holy Spirit as subordinate to both.

More precise are the following definitions:

The relevant article in Dictionary of Theology (Karl Rahner and Herbert Vorgrimler, eds.) says that the term expresses the idea that the Son and the Spirit

“do not fully possess the divine essence (Homoousion).”

Similarly, Millard Erickson, in Concise Dictionary of Christian Theology says that it is

‘the doctrine that in essence and status the Son is inferior to the Father, or the Spirit is inferior to the Father and the Son.’

Frances Young, in The Westminster Dictionary of Christian Theology, defines it as

‘any christological position which subordinates the Son to the Father in such a way as potentially to endanger his essential divinity.’

And The New Dictionary of Theology (Joseph A. Komonchak, Mary Collins, Dermot A. Lane, eds.) defines ‘subordinationism’ as

‘a view of Christ which maintains that he is not equal in substantial being with God the Father.’

Grudem (one of the most prominent advocates of the doctrine of the eternal subordination of the Son) defines subordinationism as

‘The heretical teaching that the Son was inferior or “subordinate” in being to God the Father.’ (Systematic Theology, 1st and 2nd eds.)