Team Esteem is about the self esteem of a team of people, or how they feel about the group they are in. It's something that we notice on events; some teams have a really good feel about them and enjoy each others' company and with others it's harder work. Generally most of the teams we meet on our team building days seem to have high Team Esteem. That may be because the days we run are enjoyed and looked forward to, or it may just be that teams who aren't like this don't do things like fun team building.
Whatever it is, there's no doubt that a fun team building event is great for morale and high Team Esteem.