Mana. Enchantment. Power, magical energy - on the small scale it powers Allomancy, Surgebinding, etc. On the large scale, it's the tool of pluripotence used by a Shard. Leave a large enough chunk of it alone for long enough, and it turns sentient - Splinters such as spren or seons. Shape it with a magic system and you get an effect. Each Shard seems to have distinctive Investiture, or maybe it's just the way they shape it that's different. Any Shardworld with a magic system is Invested by one or more Shards or significant Splinters. On the scale of a Shard, directed by a mind, it's a massive amount of power, enough to craft planets from the void. Shardic Investiture seems to recycle, getting used and escaping back to the Shard, but there are possible uses that attach restrictions or semipermanent expenditures to a Shard, such as Investing a Shardworld.