A drug is systemic if it enters the bloodstream, affecting the entire body. Injections, oral tablets, and transdermal patches are almost always systemic. There are some exceptions, such as injections that include vasoconstrictors (e.g., lidocaine/epinephrine solutions). Topical gels may or may not be systemic, and it can sometimes be dose-dependent (e.g., a high dose of a topical gel could go systemic while a low dose may not).
The opposite of systemic is local—affecting only a small part of the body.