Common root of "practice," "practical," and "practicum"
According to Etymonline, they all come from the Greek praktikos.
Edit with more information according to the linked site:
Practice: 15th century - Comes to English through French, Latin, and Greek.
Practical: 1600 - This comes to English through the French "practique" (from the original Greek).
Practicum: 1904 - Comes to English through "Late Latin" (from the original Greek).