What does T superscript mean in control systems and estimators? (ARMAX)

Question

We are studying ARMAX models in my Stochastic Adaptive Control course and for the life of me, I can not figure out what this represents, though it seems to be very significant. Here's a picture for context:

Answer 1

If $A$ is an $m\times n$ matrix $A = (a_{ij})_{i,j=1}^{m,n}$, then $A^T$ denotes its transpose $A^T = (a_{ji})_{i,j=1}^{n,m}$. For example, $$ \begin{pmatrix}1&2\\3&4\\5&6\end{pmatrix}^T = \begin{pmatrix}1&3&5\\2&4&6\end{pmatrix}. $$