According to the usual defintion, a functor is finitary if it preserves all filtered colimits. An important property is that a monadic category (over Set) is finitary (in other words, given by a Lawvere theory) if and only if its forgetful functor is finitary.
A slightly weaker definition is used in The Joy of Cats; there, is finitary if it maps every directed colimit in to a jointly epic diagram (but not necessarily a universal one) in . With this definition, an algebraic category is finitary if and only if its forgetful functor is finitary. The previous result also holds, since the two definitions are equivalent for monads on Set.