I strongly recommend that anyone who is interested in learning what the Catholic Church teaches, use sound, authentic definitions and terms. I highly recommend the Modern Catholic Dictionary by Father John Hardon S.J. I also strongly discourage using any secular American English (Webster’s, Collegiate etc.) or English-English dictionary (Oxford etc.), as the secular versions repeat and provide almost exclusively an anti-Catholic bias (either blatantly or subtly). This is due to the sad residue within the Englis lexicon remaining from the English Reformation and subsequent English persecution of Catholics.
The following two definitions are directly quoted from the Modern Catholic Dictionary, by Father John Hardon S.J.
“ACCIDENTS. Things whose essence naturally requires that they exist in another being. Accidents are also called the appearances, species, or properties of a thing. These may be either physical, such as quantity, or modal, such as size or shape. Supernaturally, accidents can exist, in the absence of their natural substance, as happens with the physical properties of bread and wine after Eucharistic consecration.”
“SUBSTANCE. A being whose essence requires that it exist in itself. It is an ens per se (a being by itself) or ens in se (being in itself). It is commonly distinguished from an accident, whose essence is to exist in another, that is, in a substance. (Etym. Latin substantia, that which stands under, principle, foundation.)”