Concise Bible Dictionary:
Worship that springs from the will of man and which satisfies the flesh (Col. 2:23). Ordinances commanded by man come under this category, though their observance may be thought to be acceptable to God by the worshippers.
Strong’s Dictionary of Greek Words:
Transliteration:
ethelothreskeia
Phonic:
eth-el-oth-race-ki’-ah
Meaning:
from 2309 and 2356; voluntary (arbitrary and unwarranted) piety, i.e. sanctimony