The word Rhema (pronounced "hray-mah") is the Greek word used for "word" of God in Ephesians 6:17. Rhema (Strong's Number G4487) is defined as "that which is or has been uttered by the living voice" - in other words, the spoken word of God.
Matthew Henry's Concise Commentary describes Rhema from Ephesians 6:17 in this way:
To the Christian armed for defense in battle, the apostle recommends only one weapon of attack; but it is enough, the sword of the Spirit, which is the word of God. It subdues and mortifies evil desires and blasphemous thoughts as they rise within; and answers unbelief and error as they assault from without. A single text, well understood, and rightly applied, at once destroys a temptation or an objection, and subdues the most formidable adversary.
Finally, be strong in the Lord and in his mighty power. Put on the full armor of God, so that you can take your stand against the devil’s schemes. For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace. In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. Take the helmet of salvation and the sword of the Spirit, which is the word of God. And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the Lord’s people.