Term 

Description 

Cardinality 

The number of elements in a set.
See also multiplicity.  

Classifier 

In general, a classifier is a classification of instances — it describes a set of instances that have features in common.
In modeling, classifiers are the basic nodes of Class diagrams: class, interface, structure, delegate, enum, module. Some of them can include other classifiers, or inner classifiers (see Inner Classifiers).  

Compartment 

Some model elements (basically, classes) are represented by rectangles with several compartments inside.
You can change appearance of the compartments (see Diagram Appearance options).  

Design project 

One of the two basic modeling project types: design and implementation. A design project is language-neutral. It does not contain source code.  

Diagram 

A graphical presentation of a collection of model elements, most often rendered as a connected graph of arcs (relationships) and vertices (other model elements).
The set of available diagram for a project depend on the project type.  

Implementation project 

One of the two basic modeling project types: design and implementation. An implementation project is language-specific. It includes diagrams and source code.  

Invocation specification 

Invocation specification is an area within an execution specification on a UML 2.0 Sequence Diagram. This element is not defined in the UML 2.0 specification. It is a useful tool for modeling synchronous invocations with the reply messages. A message in UML 2.0 Sequence Diagrams has its origin in an invocation specification.  

Model element 

Model element is any component of your model that you can put on a diagram.
Model elements include nodes and links between them.  

Multiplicity 

A specification of the range of allowable cardinalities that a set may assume. Multiplicity specifications may be given for association ends, parts within composites, repetitions, and other purposes. A multiplicity is a subset of the non-negative integers.
See also cardinality.  

N-ary association 

An association among three or more classes. Each instance of the association is an n-tuple of values from the respective classes.  

Shortcut 

A shortcut is a representation of an existing node element placed on the same or a different diagram.  

View filter 

A view filter is mechanism to show or hide a specific kind of model elements.
When dealing with large projects, the amount of information shown on a diagram can become overwhelming. You can selectively show or hide information.
See Using view filters.