[R] (... not defined because of singularities) in lm()

[Jia Pu]

I got the message, " Coefficients: (1 not defined because of  
singularities)", in the returned result of lm(). What does it mean?  
And where should I start investigating why it happens?

thanks.


/// Complete result of lm()
Call:
lm(formula = durationRatio ~ isHStar + isWordFinal + oneSyllToEOW +
     isInterNuclear + isIntonNuclear + isInterLP + isIntonLF +
     zeroSyllToNA + oneSyllToNA + zeroSyllToEOP + oneSyllToEOP +
     isInterNuclear:zeroSyllToEOP)

Residuals:
       Min        1Q    Median        3Q       Max
-3.031538 -0.126943 -0.002909  0.121606  4.322135

Coefficients: (1 not defined because of singularities)
                               Estimate Std. Error t value Pr(>|t|)
(Intercept)                   1.044481   0.010694  97.669  < 2e-16 ***
isHStar                      -0.055227   0.005748  -9.607  < 2e-16 ***
isWordFinal                  -0.262071   0.009982 -26.254  < 2e-16 ***
oneSyllToEOW                 -0.031313   0.009917  -3.158  0.00160 **
isInterNuclear                0.063470   0.015699   4.043 5.32e-05 ***
isIntonNuclear                0.054570   0.011711   4.660 3.21e-06 ***
isInterLP                    -0.262302   0.011868 -22.102  < 2e-16 ***
isIntonLF                    -0.160018   0.012537 -12.764  < 2e-16 ***
zeroSyllToNA                 -0.160767   0.011105 -14.477  < 2e-16 ***
oneSyllToNA                  -0.012633   0.010779  -1.172  0.24123
zeroSyllToEOP                -0.212474   0.011987 -17.726  < 2e-16 ***
oneSyllToEOP                 -0.123324   0.010998 -11.214  < 2e-16 ***
isInterNuclear:zeroSyllToEOP        NA         NA      NA       NA
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.2662 on 9650 degrees of freedom
Multiple R-Squared: 0.3957,	Adjusted R-squared: 0.395
F-statistic: 574.5 on 11 and 9650 DF,  p-value: < 2.2e-16