What is the English term to say job quitting?

Solution 1:

"Staff turnover," is what I believe you are after.

As in:

"The staff turnover was very high due to low morale."

As it says on this 2014 fact sheet:

Employee turnover

Employee turnover refers to the proportion of employees who leave an organisation over a set period (often on a year-on-year basis), expressed as a percentage of total workforce numbers.

At its broadest, the term is used to encompass all leavers, both voluntary and involuntary, including those who resign, retire or are made redundant, in which case it may be described as ‘overall’ or ‘crude’ employee turnover. It is also possible to calculate more specific breakdowns of turnover data, such as redundancy-related turnover or resignation levels, with the latter particularly useful for employers in assessing the effectiveness of people management in their organisations

^{(emphasis added)}

Solution 2:

Attrition rate is a commonly used term to describe the proportion of employees voluntarily leaving the company, through resignation or retirement.

Edit: I do not really follow why some people want a "definition" for a term so commonly used in the industry. Nonetheless, here are your links.

A factor, normally expressed as a percentage, reflecting the degree of losses of personnel or materiel due to various causes within a specified period of time.

– The Free Dictionary

Formula:

Attrition rate (%) = (Number of employees resigned during the month / Average number of employees during the month) x 100
where Average number of employees during the month = (Total number of employees at the start of the month + Total number of employees at the end of the month) / 2.

– From Wikipedia

See also this article: Infosys's attrition rate crosses 20%

There are plenty of articles like this one.

Solution 3:

Resignation :

• An oral or written statement that one is resigning a position or office:

  • He submitted his resignation

(The Free Dictionary )

A more common and formal way to express your concept is:

• The resignation rate in this company is very high,

Solution 4:

An informal term is "churn" or "churn rate". This can describe either the rate at which customers change between competing suppliers (for internet services, satellite TV networks, etc), or to workers in jobs like fast food sales or call centers, which are regarded as short-term jobs rather than long-term careers. http://www.oxforddictionaries.com/definition/english/churn-rate