This is the definition that matches my understanding. From the site:

"Containment refers to the energetic space between you and your psychotherapist. It's the atmosphere the therapist creates that conveys a sense of safety, allowing you to more comfortably move through your emotions."

My T uses that word sometimes, and I believe that is his meaning. The first time he said it, I thought he meant he wanted me to suppress my emotions but when I asked him, he pretty much said the definition above.