1 [uncountable and countable]MHMD
an act of putting a drug into someone's body using a special needle [↪ shot]:
The nurse gave me a tetanus injection.
an injection of insulin
The children hate having injections.
The only sure treatment is antibiotics, preferably by injection.
an addition of money to something in order to improve it
a massive injection of public funds
Our local football club may fold unless it gets a cash injection.
3 [uncountable and countable]
the act of forcing a liquid into something:
a fuel injection system