It follows claims the social media giant had not done enough to tackle cyberbullies or extremists in the past.
In a blog, the firm said the new language emphasised it would "not tolerate behaviour intended to harass or intimidate".
It also said it had clarified what it considered to be "abusive behaviour and hateful conduct".
This includes:
The firm promised to suspend or close the accounts of users who "cross the line into abuse".
Pressure is growing on social media firms to tackle issues such as online extremism and cyberbullying, which are said to affect millions of young Britons every year.
The Times reported last week that the Home Secretary Theresa May had suggested that new surveillance powers - unveiled under the Investigatory Powers Bill - could be used to help police to unmask anonymous cyberbullies.
In a letter to MP James Cartlidge, seen by the paper, Ms May said: "Internet connection records would update the capability of law enforcement in a criminal investigation to determine the sender and recipient of a communication."
This could include "a malicious message such as those exchanged in cyberbullying".
Twitter has been criticised for not doing enough to tackle online abuse, and in February, then-chief executive Dick Costolo admitted the company "sucks" at dealing with trolling.
In its update, however, the firm said it had introduced a range of new protections in 2015, including new tools for reporting abusive behaviour and a ban on the "promotion of terrorism".
A spokesperson for the NSPCC, a child protection charity, said Twitter's new rules were "a long overdue but very welcome step".
"We have been urging social media companies to take decisive action to curb cyberbullying, which damages the lives of many children who tell us it is almost impossible to find a safe haven from it.
"Industry is now waking-up to this pernicious behaviour and it's good to some are taking a more proactive approach to dealing with it."
Twitter has updated its rules to highlight it is taking a tougher stance against abuse.