Keep visually identical or visually similar domain names away from criminals by defensively registering them:
Homographs, also known as homoglyphs, are characters that are visually similar to each other to the point that they are easily confused with each other during a quick inspection. With the adoption of Unicode with its 1000's of additional characters beyond the ASCII range, there are far more opportunities for such homographs. In some fonts, there are characters that are entirely identical. The Unicode Consortium published a report describing this scenario.
Now that domain name registries support Internationalized Domain Names (IDN), such character confusion can be exploited with homograph spoofing and phishing attacks.
ICANN, IANA, and domain name registries have taken steps to prevent these type of attacks with various character limits when registering a name. For instance, Verisign (registry for .com and .net TLDs) prohibits characters from certain Unicode blocks and prohibits names that are comprised of characters from different Unicode blocks. IETF has proposed an IDN character mapping for web browsers that also limits what characters comprise a domain name.