1 John 5:16-17
"If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death."
Sins which are "unto death" are mortal sins; they are severe sins which require confession and repentance to restore communion with God and the Church. Minor sins are still sins: "all unrighteousness is sin", but they do not all have the same effect on the soul, on the body, or on others. If I lose my temper and swear for spilling coffee on myself, I suppose I have sinned. But this would not prevent my receiving Holy Communion or entering into the Kingdom of God in the same way that something severe like apostasy or murder would. Different types of sins.
The severe ones are rightly called mortal, for they condemn or at least endanger the soul. Calling those lesser sins "venial" when they are "not unto death" is perfectly fine. The only qualifier, which others have made, is that Orthodoxy would not necessarily have the same theological assumptions, legal codes, and penances which Roman Catholicism employs. At times we might use the same terms, but they might mean different things.
Likewise, there are also sins which are against the body, and others which are not. This designation of different types of sins also exists in the New Testament: "Every sin that a man does is without the body; but he that commits fornication sins against his own body." 1 Corinthians 6:18.