At the peak of it's power Lithuania had all the signs of Empire except it wasn't even a kingdom way back then. Should it be considered as an empire?
This is actually a common question in Lithuanian historiography which has been raised since at least the time of Antanas Smetona in the Interwar period, and the historical consensus differs.
The biggest proponent of Medieval Lithuania being an empire, Zenonas Norkus, has put forward this as his explanation for why he considers it to be so (translated to English):
Many traits of an empire put forward by Imperiological literature can be seen in Medieval Lithuanian history. First, it is wide territorial expansion in all directions, in which it was rarely faced with a superior foe. Even in the 16th century, when Lithuania had moved to "strategic defense", it did not miss an opportunity to annex Livonia, starting the Livonian War, which it could not win without losing its independence.
Second is the desire to become a hegemon in all of Eastern Europe. In international relations theory, a hegemon is a nation which takes the role of order-keeper in a certain region, continent or even the world: orders the foreign policy of their neighbours and later becomes an arbitrator in their inner conflicts, turning them into clients or vassals.
Lithuania sought this role in the former lands of the Kievan Rus since the second half of the 14th century. Vytautas sought to include even the Golden Horde itself into Lithuanian hegemony, and in the last decades of his life, that was achieved. Lithuania only abandoned its hegemonic ambitions in 1449 - which is the year when, with a treaty, Lithuania recognized Moscow as its equal.
The third differentiating trait empires share is multiculturalism, where one of the ethnocultural groups (usually a minority) takes a politically dominant role. This trait is very visible in the case of Medieval Lithuania. Balts became a minority in Lithuania during the reign of Gediminas, but it only took until the end of the 15th century for Ruthenian nobles to start to be included among the ruling elite of the state.
Fourth - the territory or an pure has a visible center of power and peripheries, usually ruled indirectly. This trait is too very visible in the case of Medieval Lithuania. Its metropoly was made up of the duchies of Trakai and Vilnius, which later became Voivodeships and gained the name "Lithuania Propria".
Its other pages were peripheries ("ukraines"), which were tied to the imperial center by stronger or weaker political ties. And finally, it's obvious that a nation cannot call itself an empire if it is not sovereign - does not have a domestic or foreign policy. Lithuania maintained its sovereignty until the year 1569. And finally, empires can only be political units which, on account of their size, surpass all of the other regional polities during its time. From all imperial traits Lithuania had, this one is the most easily visible.
If you believe in this definition, then yes, Lithuania was an empire.