In algebra, we come across two types of symbols namely constants and variables.
Constant :- A symbol having fixed numerical value is called a constant.
Ex: 3, -2, 4, 0, 1/2, -5/3
Variable or a Literal :- A symbol which can take various numerical values is called a variable or a literal.
A symbol which takes on various numerical values is called a variable or a literal
* Which is the product of a constant and a variable is also a variable.
Ex: 3x, -5y etc.
A combination of two or more variables separated by + sign or a – sin is also a variable.

Terms:
* A constant or a variable or the product of a constant and a variable is called a term.
* Product of two or more terms is also called a term.
* Every constant or variable is called a term.
Ex: 3xy ; 2x2 y
Algebraic Expressions:
A combination of constants and variables, connected by the symbols +, -, × and ÷ is called an algebraic expression.
* The several parts of an expression separated by the sign + or- are called the terms of the expression.
* A single term or a group of terms with addition and subtraction operations is called an expression.
If every term of an expression is a constant term, such expression is called numerical expression.
Every numerical expression represents a number.
The expression containing at least one algebraic term is called an algebraic expression.
Types of Algebraic Expressions:
Algebraic expressions are categorized according to the number of terms in it.
Monomial: An algebraic expression which contains only one term is called a monomial.
Binomial: An algebraic expression which contains two terms is called a binomial.
Trinomial :- An algebraic expression which contains three terms is called a trinomial.
Multinomial:- An algebraic expression which contains two or more terms is called a multinomial.
Like terms:
The terms having same literal factors (coefficient) are called like terms or similar terms.
Unlike terms:
The terms not having same literal factors are called unlike terms or dissimilar terms.