Definitions of expression:

• noun:   the act of forcing something out by squeezing or pressing

  Example: "The expression of milk from her breast"

• noun:   the expression on a person's face

  Example: "A sad expression"

• noun:   expression without words

  Example: "Tears are an expression of grief"

• noun:   the communication (in speech or writing) of your beliefs or opinions

  Example: "Expressions of good will"

• noun:   (genetics) the process of expressing a gene
• noun:   a group of symbols that make a mathematical statement
• noun:   the style of expressing yourself

  Example: "His manner of expression showed how much he cared"

• noun:   a word or phrase that particular people use in particular situations

  Example: "Pardon the expression"

• noun:   a group of words that form a constituent of a sentence and are considered as a single unit