What is the singularity error in linear regression




I have run lm in R on some data and got the following output:

There is some output like 3 not defined because of singularity.
Can someone please explain what this means??

Dealing with singularities in a linear regression model

Some of the variables are not defined because of singularity means that the variables are not linearly independent. If you remove the variables that are giving NA in the above summary, you will obtain the same result for the rest of the variables. This is because the information given by those variables is already contained in the other variables and thus redundant.
Hope this helps!