Q:

Separatists against National Army -> Civil War? 

As an European citizen, the Ukrain war happening now is worrying me. 
On side, the  official Armed Forces of Ukraine and the separatists on the other side. 
Is this a civil war?  
This is a little subject to interpretation, but I hope that you can show some historical examples in your answer, where similar conflicts are declared as civil war or something else. 
Intuitively I would say yes.  But the presence of russian fighters in the separatistic troops making this a bit unclear for me. 
Also, the fact that EU and USA endeavour to support the official Armed Forces with weapons and training is raising the term proxy war. 

A:

It is common for civil wars to involve substantial aid from foreigners.   Sometimes this aid includes genuine volunteers; sometimes it includes hired mercenaries; sometimes it includes financial support; and sometimes it includes intervention by foreign armies and/or navies. 
For example, there have been three major civil wars in what is now the United States:

During 1688 - 1691.   This war was only settled after one faction of English (in England) invited the Prince of Orange to invade England.   (Incidentally, the political turmoil allowed the Salem witch trials to get out of control.) 
During 1775 - 1781.   Most of the self-proclaimed "Patriot" troops were Americans (who had previously called themselves "Englishmen".   The "Royalist" troops were a mix of Englishmen (from England), and German "mercenaries".   Washington's continental army was trained by European volunteers.   Starting in 1777, the French provided major amounts of funding.   The French and Spanish navies fought against the English, and the Spanish unsuccessfully besieged Gibraltar.   Washington coordinated his Yorktown campaign with the French navy, and the outcome of the war depended on a French naval victory against the English.   The English continued to occupy Detroit for 14 years after the official end of the war. 
During 1861 - 1865.   The armies and navies involved in this war were American.   Both sides' diplomacy was focused on whether England and/or France might recognize the Confederacy; Lincoln made a point of not fighting a war against either England or France at the same time.  

It is common for civil wars to be instigated by émigrés returning from exile, using funds raised by other emigrants (or their descendants).   Sometimes the return of a large number of armed émigrés is indistinguishable from an invasion.   For example:

The French-sponsored Stuart "pretenders" of 1715 and 1745. 
Castro's invasion of Cuba in 1958 started with a platoon, and rapidly grew until the government fell. 
The Bay of Pigs invasion by CIA-sponsored émigrés. 

During the Cold War, the United States and the Soviet Union supported factions in many civil wars around the world.   From the points of view of the inhabitants of the affected countries, some of these wars were "wars of liberation" against colonial powers, some were civil wars, and some were invasions.   (Some were all three.)   From the points of view of the United States and the Soviet Union, most of these wars were proxy wars. 
Some of the wars involved multiple levels of proxies.   For example:

The Soviet Union supported Cuban efforts in Central America and Angola. 
The United States and South Africa supported a different faction in Angola. 
The United States and the Arab Oil States provided support and training to the mujahideen in Afghanistan, via the Chinese and Pakistanis.