A lookalike domain is any registration whose value comes from resembling another name. That covers a typo of it, a character swap that renders identically, an extra word bolted on, or the same name under a different top-level domain. The category matters more than the technique, because a single operator usually holds several variants at once and files them as one campaign.
The resemblance does the work. Nobody reads an address character by character, so a domain that matches the shape of the real one clears the only check most people perform. That is why lookalikes are the delivery layer for phishing pages, fake shops and support scams rather than an attack in themselves.
Removal depends on which kind you are looking at. A domain that infringes a name you have rights in can be pursued through the registrar's abuse channel or a UDRP complaint. One that merely resembles a name nobody owns is far harder, which is why the practical answer is to enumerate the cluster and go after the shared hosting and registrar rather than each domain in turn.
Dormant variants are still worth removing. A domain that sits parked for a year has aged, and age makes it more credible to spam filters, not less. The cheapest moment to remove one is before it has been pointed at anything.
How Fraudox handles it
Scam domain removal