Fill in the blank: With the User Directory Software Blade, you can create user definitions on a(n) ___________ Server.

The correct choice is LDAP, as it stands for Lightweight Directory Access Protocol, which is a widely used protocol for accessing and maintaining distributed directory information services over a network. The User Directory Software Blade in Check Point's security solutions specifically supports integration with LDAP servers for user authentication and management. This allows administrators to create, modify, and manage user definitions directly from the LDAP server, enabling streamlined user access and enhanced security measures.

In the context of the other options, while SecurID (which is a security technology that generates one-time passwords), NT domain (referring to Microsoft Windows NT domain for managing users), and SMTP (Simple Mail Transfer Protocol, used for sending emails) serve important roles in network security and communication, they do not function as user directory systems like LDAP does. LDAP is specifically designed for managing user information and is highly compatible with the User Directory Software Blade, making it the most relevant choice for creating user definitions within this context.