Serial.write()

Writes binary data to the serial port. This data is sent as a byte or series of bytes; to send the characters representing the digits of a number use the Serial.print() function instead.
Serial.write(val)
val a value to send as a single byte returns the number of bytes written, though reading that number is optional
void setup() {
    Serial.begin();
}

void loop() {
    Serial.write(45); // send a byte with the value 45
}