What is the definition of a property?
We call a set of languages, $P\subseteq 2^{\Sigma^*}$, a property. If you think of this subset as the set of languages who satisfy some property, then we can simply say that a language $L$ satisfies the property iff $L\in P$.

Rice theorem tells you that you can't, given a Turing machine $M$, check if $L(M)$ satisfies some non trivial property $P$, i.e. $P\neq \emptyset$, $P\nsupseteq RE$. Note that Shaull makes a distinction between semantic and syntactic properties in the comments. To the best of my knowledge, the standard definition of a property in the context of Rice's theorem is a set of languages.