With reference to Dr Frank Mobbs' letter in the September AD2000, my understanding of a Council's infallible teaching is that this does not require use of the words 'we define' or 'this is definitive'. All that is needed is that the teaching be presented in an authoritative or definitive manner.
As Father William G. Most states on the EWTN Website (under 'Faith'), 'when the Church presents some doctrine as definitive or final, it comes under [Christ's] protection: it cannot be in error; in other words it is infallible. This is true even if the Church does not use the solemn ceremony of definition. The day to day teaching of the Church throughout the world, when the Bishops are in union with each other and with the Pope and present something as definitive, this is infallible ...'.
Without considering which teachings in Vatican II fall into this category, I would argue that Councils (Bishops and Pope together) do not have to necessarily 'use the solemn ceremony of definition' in order for a declaration to be infallible.