Final answer:
The rectangular pier formed by thickening the end of a projecting wall, which appears to be supportive but is mainly decorative, is known as a pilaster.
Step-by-step explanation:
A rectangular pier or pilaster formed by thickening the end of a projecting wall is known as a pilaster. A pilaster is a rectangular column that projects partially from the wall to which it is attached; it provides the appearance of support but in fact, it is typically just for decoration. Pilasters can have various decorative elements such as fluting, which are the vertical, indented lines or grooves that can also be found on columns, and different types of capitals like Ionic with scroll shapes or Corinthian with leaf shapes.