Under II. "BODY AND SOUL BUT TRULY ONE", from the Catechism of the Catholic Church, 365, the Church teaches that:
365 The unity of soul and body is so profound that one has to consider the soul to be the "form" of the body:1 i.e., it is
because of its spiritual soul that the body made of matter becomes a
living, human body; spirit and matter, in man, are not two natures
united, but rather their union forms a single nature.
Therefore according to Catholic teaching, man's one nature is the union of spirit and matter, soul and body.
The section mentioned above continues
367 Sometimes the soul is distinguished from the spirit: St. Paul for instance prays that God may sanctify his people "wholly", with
"spirit and soul and body" kept sound and blameless at the Lord's
coming.2 The Church teaches that this distinction does not
introduce a duality into the soul.3 "Spirit" signifies that
from creation man is ordered to a supernatural end and that his soul
can gratuitously be raised beyond all it deserves to communion with
Therefore the Church teaches that while the soul is sometimes distinguished from the spirit, the distinction does not introduce a duality into the soul.
1. Cf. Council of Vienne (1312): DS 902.
2. 1 Thess 5:23.
3. Cf. Council of Constantinople IV (870): DS 657.
4. Cf. Vatican Council I, Dei Filius: DS 3005; GS 22 § 5; Humani Generis: DS 3891.
The section The soul in Christian thought in the article Soul | New advent.
Addendum after reading @PeterTurner's answer.
In the New Testament the word spirit and, perhaps, even the expression
spirit of God signify at times the soul or man himself, inasmuch as he
is under the influence of God and aspires to things above; more
frequently, especially in St. Paul, they signify God acting in man[.]
- Holy Ghost | New Advent.