In Katz and Mazur's book "Arithmetic moduli of elliptic curves" (available here), the previously undefined notion of "universal formal deformation" is stated and I fail to understand how it is defined (actually, what it even is). The first occurence of it is on page 130 (page 71 in the pdf). More precisely, here is the statement.
Let $k$ be an algebraically closed field of characteristic $p>0$, $E_0/k$ a supersingular elliptic curve, and $\mathbb{E}/W(k)[[T]]$ its universal formal deformation.
My question is: how is the object $\mathbb{E}/W(k)[[T]]$ defined? In particular, what is $W$?
If anybody could provide a definition for it or a reference, I would be really thankful.
NB: If this is any relevant, a supersingular elliptic curve was previously defined as those elliptic curves whose $p$-divisible group is, up to $k$-isomorphism, the unique $1$-parameter formal Lie group over $k$ of height $2$.