Richard Purtill proposed the following definition:
A miracle is an event (1) brought about by the power of God that is (2) a temporary (3) exception (4) to the ordinary course of nature (5) for the purpose of showing that God has acted in history. (“Defining Miracles” in In Defense of Miracles, IVP, 1997, p.72).
I have previously argued that condition (5) should be rejected, and in my last post on this subject (7/25/08), I began to examine the first condition:
(1) brought about by the power of God
I eliminated the unclear term “God” from this condition:
(1a) brought about by the power of a person who is all-powerful, all-knowing, and perfectly good.
Then I pointed out an ambiguity:
(1b) brought about directly by the action of a person who is all-powerful, all-knowing, and perfectly good.
(1c) brought about either directly or indirectly (through the actions of others who have been empowered to do so) by a person who is all-powerful, all-knowing, and perfectly good.
Contrary to my own previous comments, both (1b) and (1c) would require the existence of God (i.e. an all-powerful, all-knowing, perfectly good person) in order for a miracle to occur. A miracle, on either condition, would imply the existence of God.
However, a “temporary exception to the ordinary course of nature” could be caused by some lesser being with supernatural powers, so we would have the epistemic problem of distinguishing apparent miracles (i.e. supernatural events not brought about by God) from real miracles (supernatural events brought about by God, either directly or indirectly).
One objection that applies to both (1b) and (1c) is that the ordinary use of the word “miracle” does not imply that the event was brought about by an all-powerful, all-knowing, and perfectly good person. The ordinary use of the word “miracle” allows for such events to be brought about by beings that don’t measure up to all three of these perfections, even by deities that lack all three perfections.
Note how the word is used in the Encyclopaedia Britannica article on “Miracle”:
Belief in miraculous happenings is a feature of practically all religions, and the incidence of miracles (i.e., of belief in and reports regarding miracles) is universal, though their functions, nature, purpose, and explanations vary with the social and cultural—including theological and philosophical—context in which they appear. However inexplicable, all miracles have an explanation in the sense that they are accounted for in terms of the religious and cultural system that supports them and that, in turn, they are meant to support. Without such an accompanying—explicit or implicit—theory (e.g., the presence, activity, and intervention of such realities as gods, spirits, or magical powers), there would be no miracles in the aforementioned sense but only unexplained phenomena. 
The article speaks of “miracles” being attributed to Greek deities:
Hellenistic religion presents one of the best examples of a civilization in which miracles play a major part. The intervention of the gods in the affairs of the Homeric heroes takes place in a cosmos in which the divine and human spheres still interact.
Later Hellenistic syncretism conceived of the sublunar world as a distinct sphere, though higher powers could miraculously irrupt into it. Miraculous cures (e.g., at the sanctuary of Asclepius at Epidaurus), divine manifestations of various kinds (e.g., voices, dreams, and theophanies), and even virgin births and resurrections were widely reported. 
The gods of Hellenistic religion were not viewed as being perfectly good persons. Nor were they viewed as being all-powerful. If a miracle could be brought about by such a god, then the existence of an all-powerful, all-knowing, and perfectly good person is not a necessary condition for the occurrence of a miracle.
Another point in support of this objection is that miracles are supposed to be used as the basis for determining which religion of the many competitors is the “true religion” and which sacred book is the “true revelation”. If the word “miracle” is defined in a way that excludes polytheistic gods and religions, such as Hellenistic religion, then this appears to beg an important question, and to bias the outcome (stack the deck).
Virgin births and resurrections in Hellenistic religion are excluded from consideration before we even start investigating miracles, if we define the word “miracle” so that only those supposed supernatural interventions that were allegedly brought about by an all-powerful, all-knowing, perfectly good person are examined.
Are these objections sufficient to justify broadening the first condition to allow for miracles to be brought about by lesser beings?
(1d) brought about by the power of a god or spirit
To be continued…
 from section “Nature and significance”. Retrieved 2, 2007 from Encyclopaedia Britannica Online Library Edition: http://www.library.eb.com/eb/article-34104
 from section “Miracles in the religions of the world”. Retrieved 2, 2007 from Encyclopaedia Britannica Online Library Edition: http://www.library.eb.com/eb/article-34104