Bigotry towards, and/or prejudice against, transgender people. Includes any malicious act which may worsen an individual's gender dysphoria.
Acts considered transphobic include intentional misgendering, the use of slurs, hate crimes against a trans person, and other unsavoury violations of trans rights.
If you know someone suffering from transphobia, there are numerous dedicated hotlines and websites you can contact or visit to report a violation of LGBTQ+ rights.
https://action.aclu.org/legal-intake/report-lgbtqhiv-discrimination
https://lgbt.foundation/report
https://www.hrc.org/resources/what-to-do-if-youve-been-the-victim-of-a-hate-crime
https://www.stonewall.org.uk/help-advice/hate-crime/what-hate-crime

See also:
homophobia
acephobia
offensive
hate crime
racism
ableism