Regarding the ecclesiological status of reformed communities, you cite a dictionary. That proves that these communities can be called "church" in accordance with the English language. Linguistically, this is true. But it is wrong theology.
Christ built His Church on the foundation of Peter (Matthew 16:18) and through His apostles, who He sent to the world in His stead (John 20:21). He predicted that the church will be one despite temporal divisions (John 17:11-23; 1 Corinthians 12:12). He predicted that the apostles will be taught proper doctrine by the Holy Ghost (John 14:26). He also predicted that the Church as a whole is perfect and glorious (John 17 22-23). His apostle St. Paul taught that all be admitted to the Church (1 Timothy 2:4). His other apostle and the first pope St. Peter decreed that his office will be held by his successors in perpetuity (2 Peter 1:15). Accordingly, the marks of the Church are one, holy, catholic and apostolic.
The Protestant communities of faith variously miss at least one of these marks. The organizatonal union with Rome, incidentally, is not among the marks. Till relatively recently, the Church of England was considered a part of the true visible Church, because it maintained the apostolic succession, and despite its opposition to Rome. Since then, due to sever doctrinal drift, they lost their apostolicity, and today, with homosexual bishops they are firmly along the road to lose their holiness.
Not so with the Eastern Orthodox. Too little separates us doctrinally to deny that we have oneness of teaching. Their liturgical and monastic tradition ensures their holiness; their ecclesiology is all-inclusive and hence catholic; they have the same apostolic succession. For these reasons the Catholic Church teaches that the Orthodox are a valid church in the theological sense.
While points of disagreement exist, and many Orthodox vigorously resist all ecumenist entreaties, I am not aware of any disagreement regarding the interpretation of scripture. Personally, I rely on both Catholic and Orthodox exegesis of the scripture interchangeably.
Christ's church was built upon Himself. "That Rock was Christ." "Other foundation can no man lay than that is laid, which is Jesus Christ." (1 Cor. 3:11.) "Now therefore ye are no more strangers and foreigners, but fellow citizens with the saints, and of the household of God; and are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief comer stone." (Eph. 2:19-20.)
Peter wrote this
"As newborn babes, desire the sincere milk of the word, that ye may grow thereby: if so be ye have tasted that the Lord is gracious. To whom coming, as unto a living stone, disallowed indeed of men, but chosen of God, and precious, ye also, as lively stones, are built up a spiritual house, an holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ. Wherefore also it is contained in the scripture, Behold, I lay in Sion a chief corner stone, elect, precious: and he that believeth on him shall not be confounded. Unto you therefore which believe he is precious: but unto them which be disobedient, the stone which the builders disallowed, the same is made the head of the corner, and a stone of stumbling, and a rock of offence, even to them which stumble at the word, being disobedient: whereunto also they were appointed." (1 Pet. 2:2-8.)
"Jesus Christ himself being the chief corner stone; in whom all the building fitly framed together groweth unto an holy temple in the Lord." (Eph. 2:20-21.)