Expression : as it applies to web development technology
In programming the term expression is used to describe code that resolves to a value when you assign, evaluate, check, compare or compute values in your script.
When you use operators in any scripting language, "expression" refers to the the values and the operator that are performing the logic that will resolve to some value that your script is returning to your program.
Here are examples of expressions you might see in programming:
4 + 6
(this expression returns the sum of 4 and 6)
var1 + var2
(this expression returns the sum of whatever values that your variables represent)
var1 == var2
(this expression checks to see if the operands are equal, it returns either true or false as value)