Error: relay was not declared in the scope

Please help with the error in the following code:

#include <VirtualWire.h>
 
void setup()
{
Serial.begin(9600);
vw_set_ptt_inverted(true); // Required by the RF module
vw_setup(2000); // Transfer rate in bps
vw_set_rx_pin(4); // Receiver DATA pin connected to 2nd Arduino's digital pin 4
vw_rx_start();
pinMode(relay,OUTPUT);

Serial.begin(9600);

}
 
void loop()
{
uint8_t buf[VW_MAX_MESSAGE_LEN];
uint8_t buflen = VW_MAX_MESSAGE_LEN;
if (vw_get_message(buf, &buflen)) // Non-blocking
{
if(buf[0]=='1'){


digitalWrite(13,1);
} 
if(buf[0]=='0'){
digitalWrite(13,0);
}

}
}

//reciever code

The compiler told you exactly what's wrong: you didn't declare 'relay'.
If you don't know what declaring means, you should google it.

For example: Variables and types - C++ Tutorials

Pieter