What does individual variables mean ? Can be propositional variables? 
We know that any particular first-order language is 
  determined by its symbols. These consist of ;
$ 1-) \ $ A denumerable list of symbols called individual variables. 
  
  
*
  
*A denumerable list of symbols $(\text{not in }1)$ called individual parameters. 
  
*the connectives ; $\land ,\lor ,\lnot , \rightarrow $
  
*for each natural number $n$, a set of $n$-ary relation symbols (also 
  called predicate symbols). 
  
*for each natural number $n$, a set of $n$-ary function symbols. 
  
*the quantifiers ; $ \forall , \exists$
  
*parentheses and the comma. $( \ , \ )$


My qeustion : 
$1-) \ $ What does  individual variables mean ? Can be propositional variables?
$2-) \ $ In first-order language ; Can be definition symbol?
 A: 
1) What does individual variable mean ?

It is a term, i.e. a symbol that acts as a name for an object. 
Thus, it cannot be a propositional variables, i.e. a symbols that stands for a sentence.
Consider the simple example from first-order language of arithmetic : $(x=0)$.
In this formula $x$ must be replaced by  a number in order to give an arithmetical meaning to the formula.


2)  In first-order language, can be definition symbol ?

A definition must either introduce a term, i.e. a symbol acting as a name for an object, or a predicate letter, i.e. a symbol naming a property.
Again, examples from first order arithmetic : we start from the basic symbols of the language : $0$ (an individual constant denoting the number $\text {zero}$), the unary function $s(x)$ (the $\text {successor}$ function) and the binary function $+(x,y)$ (the $\text {sum}$ operation, abbreviated with : $(x+y)$).
With them we define the new constant $1$ as $s(0)$.
And we define the new binary predicate $<(n,m)$ (the relation $\text {less than}$, abbreviated with $(n < m)$) as follows :

$(n < m) \text { iff } \exists z \ (m=n+s(z))$.

