drush: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/drush - from package: drupal6-drush
/usr/bin/drush - from package: drush
/usr/bin/drush - from package: php-drush-drush

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/drush - from package: drush

You may install the required package with command apt-get install {package}

Error in other languages:
drush: bevel nie gevind nie
drush: comandă negăsită
drush: comando não encontrado
drush: commande introuvable
drush: command not found
drush: käsku ei ole
drush: không tìm thấy lệnh
drush: komanda nerasta
drush: Kommando nicht gefunden.
drush: kommandot finns inte
drush: komut yok
drush: nie znaleziono polecenia
drush: níor aimsíodh an t-ordú
drush: no se encontró la orden
drush: no s'ha trobat l'ordre
drush: opdracht niet gevonden
drush: parancs nem található
drush: perintah tidak ditemukan
drush: príkaz nenájdený
drush: příkaz nenalezen
drush: команда не найдена
drush: командата не е открита
drush: 命令找不到
drush: コマンドが見つかりません