According to C.B. Moss, an Anglican scholar of the early Twentieth Century, for a Church to pronounce a sacrament valid simply means it recognises that it was "performed in accordance with her law" (The Christian Faith, p.334). Similarly, he says that a sacrament is said to be invalid when "something which the Church requires is lacking" (ibid. p. 334). Moss emphasises the legal connotations of the word and even goes so far as to say "There is no such thing as absolute validity, for 'validity' means recognition by a particular society" (ibid. p. 337) While this is strictly true as a matter of verbal definition, it runs the risk of reducing validity to a merely human construction of relative and nominal significance.
However, Moss balances this perspective by also noting that Churches do not have plenary authority to arbitrarily change or set the "rules", such as by substituting different elements to bread and wine for Communion or accepting non-episcopal ordination as valid. He mentions "our Lord's institution" and what is "universally accepted" in these contexts, implying thereby that these are the reasons that certain conditions for sacramental validity are necessary in a more than legal sense. Indeed, a more satisfactory definition of sacramental validity from a Catholic perspective emphasises God's promise to offer grace when certain kinds of symbolic actions and prayers are made in accordance with His revealed design and will.
It is often assumed that if a sacrament is pronounced invalid, that this is equivalent to stating it is certain that the sacrament was no sacrament at all and ineffectual. This is incorrect. To say a sacrament is not valid is to say that God's grace, which is guaranteed if we follow his covenanted means, cannot be presumed to be present because those covenanted means were not practised. However, God can and does act outside His normal sacramental means. Therefore, in the same way we cannot assert (based on the purported sacramental action considered in itself) that the relevant grace was granted, we cannot say for certain that it was not. At least, this is the normal position taken by ecumenically informed Catholic theologians.