In Julia, you can define a default value for a function argument by using the syntax argname=defaultvalue. This allows the user to omit the argument when calling the function, in which case the default value will be used instead. Here's an example:
function greet(name="friend")
println("Hello, $name!")
end
greet() #outputs "Hello, friend!"
greet("Alice") #outputs "Hello, Alice!"
In the above example, the greet() function has a default value of "friend" for the name argument. If the user calls greet() without an argument, the function will output "Hello, friend!". If the user calls greet("Alice"), the function will output "Hello, Alice!".