a mobile, armored vehicle capable of things like transporting infantry and serving as a mobile defensive position to fall back to, engaging enemies with direct or indirect heavy fire, serving as a command and control vehicle, etc
the line between "tanks" and IFV's seems to be getting blurrier, because the idea of a relatively heavily armored, big-gun, "breakthrough" tank really is a relic of cold war strategy. the past few decades have seen tank designs evolve to become more mobile and even capable of carrying infantry. at the same time, 'infantry fighting vehicles' are becoming more armored and carrying higher-caliber main weapons.
tactically, an armored vehicle appearing on a battlefield is still advantageous: you've immediately aggro'd the enemy forces and allowed your own infantry to breathe for a second while the enemy figures out how to deal with the armor. you could bait out hidden antitank elements. you could mount some powerful optics and a radio, and you can have a commander remain close to the elements they are directing, while relatively protected. you could use heavier armor as line-of-sight howitzers to clear urban environments.
what's changed is it's now a real bad idea to send in heavy armor as a 'spear tip' in an invasion, you want to support them with infantry, and at the point where you're forced to mix armor and infantry, your armor will begin to resemble more infantry-friendly designs. the example probably most familiar with folks is the israeli merkava tank - for all intents and purposes it's a 'main battle tank' but it can also carry 6 passengers in a rear fighting compartment, which i imagine allows for some really flexible fire-and-maneuvering.
in the context of the russia-ukraine thing, all of this flies out the window; i have no idea what the russians are doing. partly because of the fog of war but partly, from what i can gather, because their military was kinda dogshit going into this conflict. i swear to god it's like they've unlearned stuff since 1988.
[EDIT] It also doesn't help if the opponent has several big friends with immense reconnaissance capabilities that are telling them exactly where the tanks are at all times, allowing them to pick the best spots for ambush to make them much, much more effective than they normally would be.
What Putin has used is the "Armoured Assault", not really effective since Germany's Panzerwaffe. It's the tactic not the equipment that's out of date.
From [1]
> circa 1934: "Basil Liddell Hart, now concluded that the defense had become much the stronger form of war and that armoured offensives would come to grief against a properly organized enemy."
[1] https://www.britannica.com/topic/tactics/The-armoured-offens...
Javelin costs (according to google) 178K and 78K for rocket, while T-14 tank $3.7M.. sounds like dime on a dollar. war is about cost & time to build something and it's clear that tanks loose here.
And it's super simple to train someone to launch a missile, while a tank crews needs to be trained WAY LONGER.
yes, terrifying for civilians, but ineffective for prepared adversary on a 21st century battlefield.
Imo tank battles as in 20th century will remain only as part of World of Tanks. Battleships are next as well... because hypersonic missiles.