Found in Renaud Wildura’s excellent guide to the use of Java’s final keyword, a quote from Design Patterns:

Aggregation implies that one object owns or is responsible for another object. Generally we speak of an object having or being part of another object. Aggregation implies that an aggregate object and its owner have identical lifetimes.

Acquaintance implies that an object merely knows of another object. Sometimes acquaintance is called "association" or the "using" relationship. Acquainted objects may request operations of each other, but they aren't responsible for each other. Acquaintance is a weaker relationship than aggregation and suggests much looser coupling between objects. It's easy to confuse aggregation and acquaintance, because they are often implemented in the same way. Ultimately, acquaintance and aggregation are determined more by intent than by explicit language mechanisms. Aggregation relationships tend to be fewer and more permanent than acquaintance. Acquaintance, in contrast, are made and remade more frequently, sometimes existing only for the duration of an operation. Acquaintances are more dynamic as well, making them more difficult to discern in the source code.