I want to know what is orthostatic syncope?

That means losing consciousness when you stand up suddenly. There can be many causes of this, from Anemia to Dehydration, etc.

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When you sit up real fast ...your blood pressure doesn't always catch up and your brain doesn't get the circulation it needs so you get dizzy. Some people have serious blood pressure problems and this syncope can be a sign of heart failure of some kind...It can also occur if you are dehydrated. It is all about hemodynamics.
NO orthostati syncope is not always passing out...it is also dizziness...you can have this conditon without passing out. You should get up slowly if you suffer from this condition.

A drop in systolic blood pressure upon standing of greater than 20 mmHg is define as postural hypotension.

Postural hypotension should be investigated, especially if the patient is symptomatic.

Usually the patient will complain of blackouts and dizzy turns, the result of impaired cerebral perfusion.

That would be when you stand up either from a sitting or lying position, rapidly, and your blood pressure drops (bottoms out) and you pass out. Syncope is a fancy word for passing out.

a sudden brief loss of consciousness with loss of postural tone.

Faint when you stand up related to a drop in your blood pressure

it means you have a "quick change in blood pressure" ......difference between lying down, sitting and standing...........it means your blood pressure medication needs adjusting....
in the mean time..........get up slowly.......from lying to sitting......take a few minute....
from sitting to standing.....take a few more minutes for your BP to regulate.......or you will become light headed/dizzy and fall