1) An estimation that is too low 2) Approximation 3) Assign too low a value to 4) Estimate 5) Estimation 6) Idea 7) Make too low an estimate of 8) Sell short 9) Underestimation 10) Underrating 11) Underreckoning 1) Err 2) Lowball 3) Miscalculate 4) Misjudge 5) Sell short 6) Takelightly 7) Underestimation 8) Underrate 9) Underrating 10) Underreckoning 11) Undervalue 
• (n.) The act of underestimating; too low an estimate. • (v. t.) To set to/ low a value on; to estimate below the truth.
[n] - an estimation that is too low 2. [v] - make too low an estimate of
An estimate that is less than the exact answer 
verb make too low an estimate of; `he underestimated the work that went into the renovation`; `Don`t underestimate the danger of such a raft trip on this river`underestimation noun an estimation that is too low; an estimate that is less than the true or actual value
[NAEP glossary] make too low an approximation of