The T-square or T-cross is an aspect pattern in the shape of a triangle made of square and opposition aspects. It indicates difficult conflict but also dynamic and motivating energy.
Aspect configuration. Two planets in Opposition and a third planet mid-way between them forming a Square to both sides. Looks like a "right angle triangle." A stressful but motivating energy. There is a lot of tension in the T-square, but also very dynamic energy. It often represents issues, behaviour and obstacles that need to be problem-solved. This can generate enormous creative friction that gives energy, drive and motivation to goals and ambitions in the person's life. In Huber astrology this is called an Achievement Triangle. See Bruno and Louise Huber's book Aspect Pattern Astrology for excellent information about aspect patterns.
The T-square pattern is called an Achievement Triangle in Huber astrology.