If you open the expressions language menu on an AE layer or effect property, you’ll find a list of submenus containing bits of expressions syntax. These bits of expressions language like linear(), valueAtTime(t), are often referred to as expressions, but they might more accurately be called “expression elements” – or the basic building blocks designers use along with Java Script elements and operators to construct more complex custom expressions. With a few exceptions, this catalog only deals with the more complex custom expressions that are ready to plug and play with minor tweaks.  If you’re looking for a comprehensive guide to all the expression elements listed in the AE expressions menu, we recommend checking out the After Effects Expressions Reference Guide.