C preprocessor, 15 12 bytes

#define f 1/

Try it online!

16 13 bytes, if it needs to handle integer input also:

#define f 1./

So you can call it with f(3) instead of f(3.0).

Try it online!

Thanks to @hvd for golfing 3 bytes!

C, 15 12 bytes

#define f 1/

Try it online!

16 13 bytes, if it needs to handle integer input also:

#define f 1./

So you can call it with f(3) instead of f(3.0).

Try it online!

Thanks to @hvd for golfing 3 bytes!

C preprocessor, 15 bytes

#define f(x)1/x

Try it online!

16 bytes, if it needs to handle integer input also:

#define f(x)1./x

So you can call it with f(3) instead of f(3.0).

Try it online!

C preprocessor, 15 12 bytes

#define f 1/

Try it online!

16 13 bytes, if it needs to handle integer input also:

#define f 1./

So you can call it with f(3) instead of f(3.0).

Try it online!

Thanks to @hvd for golfing 3 bytes!

C preprocessor, 15 bytes

#define f(x)1/x

Try it online!

C preprocessor, 15 bytes

#define f(x)1/x

Try it online!

16 bytes, if it needs to handle integer input also:

#define f(x)1./x

So you can call it with f(3) instead of f(3.0).

Try it online!