siRNA, RNAi words I understand but can't verbally define...therfore:

siRNA-small interfering RNA or short interfering RNA
Short (or small) interfering RNA; this is a short 21-23 nt RNA duplex involved in eliciting the RNAi response in mammalian cells.
www.invitrogen.com/content.cfm

Small strands of RNA that interfere with the translation of messenger RNA. Double stranded SiRNA works better than single stranded SiRNA. They are usually 21 to 23 nucteotides long. They bind to the complementary portion of the target messenger RNA and tag it for degradation. siRNA's effect of inhibiting gene expression is commonly known as gene "silencing".
www.alumni.ca/~blan4a0/glossary.html

Small interfering RNAs, which target (in a sequence-specific manner) endogenous RNAs for degradation, thereby reducing the function of a gene.
www.nature.com/nrg/journal/v5/n9/glossary/nrg1427_glossary.html

Small interfering RNA (first found in 1999 Science. 1999 Oct 29;286(5441):950-2. ) Related terms RNAi, Post-Transcriptional Gene Silencing PTGS (found in 1999)

RNAi – RNA inhibitor
RNA interference. A natural cellular mechanism by which RNA is recognized as “foreign” due to its existence in a double-stranded form. This results in the degradation of the double-stranded RNA, along with single-stranded RNA having the same sequence.www.sirna.com/glossary.html

RNA interference is a recently discovered functional tool. This is a phenomenon where an RNA introduced to a cell ultimately causes the degradation of the complementary cellular mRNA, and leads to the knockdown of gene activity.www.invitrogen.com/content.cfm

The use of long double stranded chains of ribonucleic acids to interfere and thus silence the expression of target geneswww.vastox.com/research/glossary.html

In molecular biology, RNA interference (RNAi) is a mechanism in which the presence of small fragments of double-stranded RNA (dsRNA) whose sequence matches a given gene interferes with the expression of that gene. en.wikipedia.org/wiki/RNAi

RNA interference: A gene silencing phenomenon whereby double-stranded RNAs trigger the specific degradation of a homologous mRNA. The specific dsRNAs are processed into small interfering RNA (siRNA) which serves as a guide for cleavage of the homologous mRNA in the RNA- induced silencing complex (RISC). Useful as an application for specific suppression of an individual gene.www.uark.edu/ua/ricecap/ricecapgloss.htm