That which cannot be otherwise or is unlikely to be otherwise. 

Absolute Necessity.— That which cannot in any way be otherwise,e.g., that ±3 is the square root of 9 or that a rational animal as a rational animal has the power of reason or that something cannot both exist and not exist at the same time and in the same respect. 

Hypothetical (or Conditional) Necessity.— That which is not necessary in itself but only on the basis of some condition. E.g., it is not necessary that I travel north unless some destination that I wish to reach lies in that direction. 

Moral Necessity.— That which cannot be otherwise if some good is to be achieved and some evil avoided. Thus, strictly speaking, it can be otherwise but should not be so. An example of this would be obedience to the natural moral law.  This is a species of hypothetical necessity. 

Practical Necessity.— That which might be achieved in the absence of some factor but is unlikely to be achieved in its absence,e.g., that one should study a book in order to pass an exam on the book.  This is a species of hypothetical necessity.