A conditional clause or protasis (plural: protases) is a clause which has a condition. These are sometimes pedagogically known as an if clause, since the most common construction includes the word "if" - but this is not necessary - and so technically an "if clause" is a type of conditional clause.
Some examples in English include:
|"If it rains today,..."||If clause|
|"Unless something goes wrong,..."||Unless clause|
|"Should I go home,..."||conditional should|
|"Were I rich,..."||Inverted conditional with be|
|"Had I a lot of money,..."||Inverted conditional with lexical have|
|"Had I gone to the park,..."||Inverted conditional with auxiliary have|
Note that often this uses the subjunctive mood (though sometimes this is ignored). The phrase "if clause", if applied to all of these ways is somewhat of a misnomer, but that may be accurate if used to only refer to clauses with "if" in. A conditional clause is usually accompanied by a consequential clause.
Roughly, it can be said that a conditional clause using the present simple is a first conditional-type clause, one using the past simple is a second conditional-type clause and one using the past perfect is a third conditional-type clause.