Functional integral

Question

In the path integral formula $$\psi(x,t)=\frac{1}{Z}\int_{{\bf x}(0)=x}\mathcal{D}{\bf x}e^{iS[{\bf x},\dot{\bf x}]}\psi_0({\bf x}(t))$$ $\mathcal{D}{\bf x}$ denotes integration over all paths with ${\bf x}(0)=x$ and the integral is called a functional integral. I tried to find a definition of a functional integral and found the wikipedia article (https://en.wikipedia.org/wiki/Functional_integration) not helpful at all. Can someone please give me a clear and concise definition of a functional integral? Many thanks in advance!