Kon'nichiwa. (こんにちは。) "Hello."
The wa in kon'nichiwa is written は ("ha") and not わ ("wa"), because this wa was originally a particle, since this word used to be part of a full phrase. Since then, the rest of the phrase got dropped, and only the kon'nichiwa part became a greeting. For the same reason, spell konbanwa, meaning, "Good evening," as こんばんは