started with the word : (colon) and ends with the word ; (semi-colon). For example,

: X DUP 1+ . . ;

will compile the word X, and makes the name findable in the dictionary. When executed by typing 10 X at the console this will print 11 10.

Most Forth systems include an assembler to write words using the processor's facilities. Forth assemblers often use a reverse Polish