When looking at websites from other teachers, many refer themselves as "sifu". At Wudang we would call teacher Mike "Mike" (informal) or "Mike Laoshi" (formal). So What is the meaning of the word "sifu"? As far as I know, there are actually 2 interpretations. The first one is that of the language: "Si" means teach and "fu" means father, it refers to the fact that good martial arts teachers not only teach a student how to fight (health and strengh) , but also how not to fight (mental and moral). A good teacher is like a good father who would point his pupil to walk the right path. In Ancient times it is not uncommon for the teacher to take orphanes off the street and raises them as his own children. Thus in a Kung Fu school, the atmosphere is like one big family.
Second interpretation is that there are 2 ways calling a teacher "sifu", the first way is that of the outsider who whishes to respect the teacher and by referring him as "sifu", the word establish the position of the "master". But on the same time, the outsider is not connected with the word (he is not family) and thus "sifu" used in this context is only a polite form of addressing the teacher (e.g. in Hong Kong, you can address a chef cook with "sifu").
For the student it is a different story, to address his teacher as "sifu" means he is addressing him as his father, which means he accepts his authority and wisdom. It also means to have loyalty as one would have for his/her father. However, there were times teachers have to open up their schools for everyone or it is not always possible to find the right student (in mind or skill), so the teacher usually keeps this relation with a very small group of selected students. When a student is chosen to enter this circle of bond, a ritual (called: "bai si" in Cantonese) will take place and the student becomes a real follower of the teacher. He/she is then an "indoor" student.
As long as you are not an "indoor" student, it is not proper to call your teacher "sifu", due to the weight of the word. The word "laoshi" means teacher (like school teacher) and is much more neutral in that aspect.
The only exception I have seen is that "sifu" is widely used in places like Hong Kong. Regardless you are a stranger, an outdoor- or indoor student, you still use the word "sifu". It is not wrong, as different areas have different habits or rules (so if you practise Southern styles like "Hung Gar", "Choi Ly Fut" or "Wing Chun", it is normal to address your teacher as "sifu" regardless you are an indoor student or not).
Do not confuse "sifu" with "sijou", "sijou" means grandmaster and is extremely high in rank. The teacher of your teacher is your sijou. E.g: Grandmaster Wang Chieh is our "sijou". If you have students you either use "sifu" or laoshi (the pronunciation of laoshi is mandarin whereas the pronuntiation of sifu is cantonese), it is not correct to use sijou when your students aren't official teachers.
Note: Respect and face value go hand in hand within the Chinese culture, which means that it is proper to address teacher Mike as laoshi when other Chinese are around (in case you wonder). As with the way Chinese interpretation of family value: A good son/daughter reflects the goodness of the parents.
(picture: Teacher Mike Martello and his true "sifu" Grandmaster Wang Chieh practising push hands)