Intercession

Intercession, Intercessor

intercession = mediation, coming between, joining together

intercessor = one who mediates, who brings together, or acts on behalf of

Expanded entry from Glossary "Glossary of terms "Intercession, intercessor"

Definition

"to intercede" or "come between", or a person who intercedes or comes between
- inter- (between) + cedere (Latin for "to go")
  - cedere from PIE *ked- "to go, yield"
- thus" go between"
  - = "a mediator"

Purpose

intercession = prayer for intercession is usually for that of Christ, the Blessed Virgin Mary, and the Saints
intercession is generally understood as that of Christ:

Intercession is a prayer of petition which leads us to pray as Jesus did. He is the one intercessor with the Father on behalf of all men, especially sinners.¹¹² He is "able for all time to save those who draw near to God through him, since he always lives to make intercession for them."¹¹³ The Holy Spirit "himself intercedes for us . . . and intercedes for the saints according to the will of God." (CCC 2634)

from CCC Glossary: "A form of prayer of petition on behalf of others. The prayer of intercession leads us to pray as Christ, our unique Intercessor, prayed (2634):

from CCC 2635:

Since Abraham, intercession - asking on behalf of another has been characteristic of a heart attuned to God's mercy. In the age of the Church, Christian intercession participates in Christ's, as an expression of the communion of saints. In intercession, he who prays looks "not only to his own interests, but also to the interests of others," even to the point of praying for those who do him harm.

A powerful example of prayer for intercession comes in the FIDEI DEPOSITUM, printed in the Introduction to the Catechism of the Catholic Church by Saint Pope John Paul II (p. 6)

At the conclusion of this document presenting the Catechism of the Catholic Church, I beseech the Blessed Virgin Mary, Mother of the Incarnate Word and Mother of the Church, to support with her powerful intercession the catechetical work of the entire Church on every level, at this time when she is called to a new effort of evangelization. May the light of the true faith free humanity from the ignorance and slavery of sin in order to lead it to the only freedom worthy of the name (cf. Jn 8:32): that of life in Jesus Christ under the guidance of the Holy Spirit, here below and in the Kingdom of heaven, in the fullness of the blessed vision of God face to face (cf. 1 Cor 13:12; 2 Cor 5:6-8)!

Intercession origins in Scripture

John 2:1

My children, I am writing this to you so that you may not commit sin. But if anyone does sin, we have an Advocate with the Father, Jesus Christ the righteous one.

Rom 8:34

Who will condemn? It is Christ [Jesus] who died, rather, was raised, who also is at the right hand of God, who indeed intercedes for us.

1 Tim 2:5-6:

For there is one God. 
There is also one mediator between God and the human race, Christ Jesus, himself human, 
who gave himself as ransom for all.

Heb 7:25

Therefore, he is always able to save those who approach God through him, since he lives forever to make intercession for them

see Intercession | Catholic Answers Encyclopedia